Form Submit Accepts JSON Responses (#7139)
Form Submit Accepts JSON Responses (#7139)

Part 2 to fixing #7119 ([Part 1](https://github.com/ampproject/amphtml/pull/7138)).
gumgum/amphtml,Adtoma/amphtml,yandex-pcode/amphtml,spotxchange/amphtml,engtat/amphtml,zhangsu/amphtml,ampproject/amphtml,prateekbh/amphtml,Mobusi-ads/amphtml,camelburrito/amphtml,jpettitt/amphtml,wjfang/amphtml,DistroScale/amphtml,BenDz/amphtml,yandex-pcode/amphtml,Mobusi-ads/amphtml,erwinmombay/amphtml,lannka/amphtml,honeybadgerdontcare/amphtml,taboola/amphtml,avimehta/amphtml,dotandads/amphtml,voyagegroup/amphtml,danielrozenberg/amphtml,lannka/amphtml,JuliusThms/amphtml,keithwrightbos/amphtml,camelburrito/amphtml,yieldbot/amphtml,clawr/amphtml,jpettitt/amphtml,jmadler/amphtml,koshka/amphtml,clawr/amphtml,fullspeed-sp/amphtml,honeybadgerdontcare/amphtml,src-code/amphtml,smartadserver/amphtml,chaveznvg/amphtml,sklobovskaya/amphtml,Gregable/amphtml,alanorozco/amphtml,voyagegroup/amphtml,clawr/amphtml,Adtoma/amphtml,kmh287/amphtml,DistroScale/amphtml,erwinmombay/amphtml,jmadler/amphtml,voyagegroup/amphtml,danielrozenberg/amphtml,Gregable/amphtml,vmarteev/amphtml,dotandads/amphtml,cvializ/amphtml,media-net/amphtml,yandex-pcode/amphtml,engtat/amphtml,chaveznvg/amphtml,rajkumarsrk/amphtml,mhc-gh/amphtml,adup-tech/amphtml,widespace-os/amphtml,burtcorp/amphtml,gumgum/amphtml,cvializ/amphtml,alanorozco/amphtml,DAC-f1h-dev/amphtml,kmh287/amphtml,jridgewell/amphtml,DarXector/amphtml,BenDz/amphtml,nexxtv/amphtml,zhucl/amphtml,Adtoma/amphtml,Adtoma/amphtml,kamtschatka/amphtml,nexxtv/amphtml,widespace-os/amphtml,gumgum/amphtml,Mobusi-ads/amphtml,bpaduch/amphtml,zhangsu/amphtml,honeybadgerdontcare/amphtml,lannka/amphtml,laterpay/amphtml,media-net/amphtml,sklobovskaya/amphtml,lannka/amphtml,JuliusThms/amphtml,clawr/amphtml,sharethrough/amphtml,sriramkrish85/amphtml,Adtoma/amphtml,yieldmo/amphtml,media-net/amphtml,ecoron/amphtml,rajkumarsrk/amphtml,pietergreyling/amphtml,sriramkrish85/amphtml,burtcorp/amphtml,ampproject/amphtml,kamtschatka/amphtml,jdelhommeau/amphtml,techhtml/amphtml,aghassemi/amphtml,src-code/amphtml,taboola/amphtml,nexxtv/amphtml,DarXector/amphtml,taboola/amphtml,comScore/amphtml,koshka/amphtml,laterpay/amphtml,engtat/amphtml,keithwrightbos/amphtml,burtcorp/amphtml,DAC-f1h-dev/amphtml,kixer-com/amphtml,dotandads/amphtml,vmarteev/amphtml,adiant/amphtml,sriramkrish85/amphtml,sortable/amphtml,rlugojr/amphtml,sklobovskaya/amphtml,taboola/amphtml,tlong2/amphtml,tlong2/amphtml,yieldmo/amphtml,rsimha-amp/amphtml,rajkumarsrk/amphtml,JuliusThms/amphtml,spotxchange/amphtml,kmh287/amphtml,techhtml/amphtml,smartadserver/amphtml,sebastianbenz/amphtml,shawnbuso/amphtml,prateekbh/amphtml,widespace-os/amphtml,bpaduch/amphtml,DarXector/amphtml,src-code/amphtml,JuliusThms/amphtml,sharethrough/amphtml,honeybadgerdontcare/amphtml,voyagegroup/amphtml,jpettitt/amphtml,shawnbuso/amphtml,engtat/amphtml,widespace-os/amphtml,rsimha-amp/amphtml,gumgum/amphtml,tlong2/amphtml,aghassemi/amphtml,jridgewell/amphtml,rajkumarsrk/amphtml,adup-tech/amphtml,pedjoni8/amphtml,kamtschatka/amphtml,sharethrough/amphtml,luciancrasovan/amphtml,techhtml/amphtml,sharethrough/amphtml,yandex-pcode/amphtml,honeybadgerdontcare/amphtml,keithwrightbos/amphtml,cory-work/amphtml,rsimha-amp/amphtml,sebastianbenz/amphtml,jmadler/amphtml,mhc-gh/amphtml,cvializ/amphtml,luciancrasovan/amphtml,fullspeed-sp/amphtml,jmadler/amphtml,BenDz/amphtml,sharethrough/amphtml,BenDz/amphtml,bpaduch/amphtml,yieldmo/amphtml,Gregable/amphtml,sklobovskaya/amphtml,fullspeed-sp/amphtml,pedjoni8/amphtml,zhouyx/amphtml,koshka/amphtml,sortable/amphtml,media-net/amphtml,cvializ/amphtml,camelburrito/amphtml,wjfang/amphtml,spotxchange/amphtml,honeybadgerdontcare/amphtml,jridgewell/amphtml,jridgewell/amphtml,yieldbot/amphtml,vmarteev/amphtml,alanorozco/amphtml,arealit/amphtml,comScore/amphtml,danielrozenberg/amphtml,widespace-os/amphtml,spotxchange/amphtml,taboola/amphtml,tlong2/amphtml,nexxtv/amphtml,DAC-f1h-dev/amphtml,luciancrasovan/amphtml,aghassemi/amphtml,smartadserver/amphtml,ecoron/amphtml,zhouyx/amphtml,Mobusi-ads/amphtml,yieldbot/amphtml,spotxchange/amphtml,rlugojr/amphtml,chaveznvg/amphtml,koshka/amphtml,gumgum/amphtml,powdercloud/amphtml,JuliusThms/amphtml,burtcorp/amphtml,powdercloud/amphtml,erwinmombay/amphtml,pedjoni8/amphtml,wjfang/amphtml,vmarteev/amphtml,DarXector/amphtml,engtat/amphtml,chaveznvg/amphtml,jdelhommeau/amphtml,yieldmo/amphtml,powdercloud/amphtml,luciancrasovan/amphtml,Gregable/amphtml,DistroScale/amphtml,ampproject/amphtml,DAC-f1h-dev/amphtml,adup-tech/amphtml,adiant/amphtml,DistroScale/amphtml,tlong2/amphtml,wjfang/amphtml,comScore/amphtml,jpettitt/amphtml,cvializ/amphtml,shawnbuso/amphtml,danielrozenberg/amphtml,danielrozenberg/amphtml,arealit/amphtml,donttrustthisbot/amphtml,kixer-com/amphtml,ecoron/amphtml,lzanol/amphtml,sriramkrish85/amphtml,smartadserver/amphtml,kmh287/amphtml,rlugojr/amphtml,techhtml/amphtml,comScore/amphtml,Mobusi-ads/amphtml,laterpay/amphtml,keithwrightbos/amphtml,avimehta/amphtml,src-code/amphtml,kmh287/amphtml,rajkumarsrk/amphtml,alanorozco/amphtml,sortable/amphtml,fullspeed-sp/amphtml,rlugojr/amphtml,prateekbh/amphtml,shawnbuso/amphtml,sebastianbenz/amphtml,jridgewell/amphtml,mhc-gh/amphtml,bpaduch/amphtml,taboola/amphtml,donttrustthisbot/amphtml,alanorozco/amphtml,jdelhommeau/amphtml,prateekbh/amphtml,voyagegroup/amphtml,gumgum/amphtml,alanorozco/amphtml,zhucl/amphtml,ampproject/amphtml,rsimha-amp/amphtml,ampproject/amphtml,adup-tech/amphtml,ADITION/amphtml,jmadler/amphtml,powdercloud/amphtml,rsimha-amp/amphtml,zhangsu/amphtml,aghassemi/amphtml,adiant/amphtml,ecoron/amphtml,prateekbh/amphtml,sklobovskaya/amphtml,BenDz/amphtml,powdercloud/amphtml,bpaduch/amphtml,sebastianbenz/amphtml,ADITION/amphtml,powdercloud/amphtml,clawr/amphtml,jmadler/amphtml,mhc-gh/amphtml,kamtschatka/amphtml,pietergreyling/amphtml,pedjoni8/amphtml,cory-work/amphtml,nexxtv/amphtml,avimehta/amphtml,lannka/amphtml,Gregable/amphtml,mhc-gh/amphtml,engtat/amphtml,jdelhommeau/amphtml,keithwrightbos/amphtml,laterpay/amphtml,camelburrito/amphtml,zhouyx/amphtml,ecoron/amphtml,clawr/amphtml,comScore/amphtml,zhouyx/amphtml,src-code/amphtml,erwinmombay/amphtml,voyagegroup/amphtml,camelburrito/amphtml,yieldbot/amphtml,lzanol/amphtml,avimehta/amphtml,kixer-com/amphtml,fullspeed-sp/amphtml,ampproject/amphtml,rajkumarsrk/amphtml,zhangsu/amphtml,DistroScale/amphtml,arealit/amphtml,cory-work/amphtml,koshka/amphtml,dotandads/amphtml,nexxtv/amphtml,DarXector/amphtml,BenDz/amphtml,prateekbh/amphtml,mhc-gh/amphtml,ampproject/amphtml,shawnbuso/amphtml,Gregable/amphtml,kmh287/amphtml,jpettitt/amphtml,fullspeed-sp/amphtml,Gregable/amphtml,laterpay/amphtml,gumgum/amphtml,jpettitt/amphtml,dotandads/amphtml,burtcorp/amphtml,ampproject/amphtml,camelburrito/amphtml,wjfang/amphtml,dotandads/amphtml,pedjoni8/amphtml,yandex-pcode/amphtml,arealit/amphtml,adiant/amphtml,zhucl/amphtml,pietergreyling/amphtml,pedjoni8/amphtml,lannka/amphtml,wjfang/amphtml,sortable/amphtml,kixer-com/amphtml,keithwrightbos/amphtml,yandex-pcode/amphtml,zhouyx/amphtml,sortable/amphtml,aghassemi/amphtml,vmarteev/amphtml,media-net/amphtml,rsimha-amp/amphtml,wjfang/amphtml,spotxchange/amphtml,media-net/amphtml,bpaduch/amphtml,nexxtv/amphtml,vmarteev/amphtml,donttrustthisbot/amphtml,sklobovskaya/amphtml,ADITION/amphtml,jpettitt/amphtml,kixer-com/amphtml,keithwrightbos/amphtml,ADITION/amphtml,widespace-os/amphtml,danielrozenberg/amphtml,yieldmo/amphtml,adiant/amphtml,kixer-com/amphtml,yieldbot/amphtml,avimehta/amphtml,vmarteev/amphtml,camelburrito/amphtml,voyagegroup/amphtml,sriramkrish85/amphtml,danielrozenberg/amphtml,zhucl/amphtml,zhangsu/amphtml,arealit/amphtml,DAC-f1h-dev/amphtml,cory-work/amphtml,rsimha-amp/amphtml,powdercloud/amphtml,jpettitt/amphtml,mhc-gh/amphtml,mhc-gh/amphtml,adup-tech/amphtml,yieldmo/amphtml,burtcorp/amphtml,sortable/amphtml,yandex-pcode/amphtml,rlugojr/amphtml,chaveznvg/amphtml,sharethrough/amphtml,pietergreyling/amphtml,cory-work/amphtml,donttrustthisbot/amphtml,sebastianbenz/amphtml,media-net/amphtml,ADITION/amphtml,comScore/amphtml,techhtml/amphtml,zhouyx/amphtml,taboola/amphtml,alanorozco/amphtml,Gregable/amphtml,arealit/amphtml,sklobovskaya/amphtml,jridgewell/amphtml,zhucl/amphtml,engtat/amphtml,Adtoma/amphtml,smartadserver/amphtml,clawr/amphtml,pietergreyling/amphtml,shawnbuso/amphtml,kixer-com/amphtml,kamtschatka/amphtml,cvializ/amphtml,lzanol/amphtml,lzanol/amphtml,tlong2/amphtml,danielrozenberg/amphtml,sebastianbenz/amphtml,donttrustthisbot/amphtml,ecoron/amphtml,jridgewell/amphtml,donttrustthisbot/amphtml,chaveznvg/amphtml,rlugojr/amphtml,bpaduch/amphtml,shawnbuso/amphtml,erwinmombay/amphtml,koshka/amphtml,DAC-f1h-dev/amphtml,yieldbot/amphtml,adup-tech/amphtml,luciancrasovan/amphtml,techhtml/amphtml,laterpay/amphtml,alanorozco/amphtml,pietergreyling/amphtml,kamtschatka/amphtml,aghassemi/amphtml,lzanol/amphtml,zhucl/amphtml,comScore/amphtml,zhangsu/amphtml,rajkumarsrk/amphtml,adiant/amphtml,DistroScale/amphtml,jdelhommeau/amphtml,arealit/amphtml,jdelhommeau/amphtml,smartadserver/amphtml,erwinmombay/amphtml,avimehta/amphtml,cory-work/amphtml,yieldmo/amphtml,src-code/amphtml,lannka/amphtml,honeybadgerdontcare/amphtml,Mobusi-ads/amphtml,sriramkrish85/amphtml,lzanol/amphtml,donttrustthisbot/amphtml,luciancrasovan/amphtml,zhucl/amphtml,DarXector/amphtml,JuliusThms/amphtml,cory-work/amphtml,media-net/amphtml,prateekbh/amphtml,adiant/amphtml,luciancrasovan/amphtml,zhouyx/amphtml