net/http: export the "new" error codes from RFC 6585
net/http: export the "new" error codes from RFC 6585

These were proposed in the RFC over three years ago, then proposed to
be added to Go in https://codereview.appspot.com/7678043/ 2 years and
7 months ago, and the spec hasn't been updated or retracted the whole
time.

Time to export them.

Of note, HTTP/2 uses code 431 (Request Header Fields Too Large).

Updates #12843

Change-Id: I78c2fed5fab9540a98e845ace73f21c430a48809
Reviewed-on: https://go-review.googlesource.com/15732
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,odeke-em/go,linux-on-ibm-z/go,josharian/go,derekmarcotte/go,CAFxX/go,suncycheng/go,cyberroadie/go,malvira/go,derekmarcotte/go,golang/go,fjballest/golang,odeke-em/go,akutz/go,theif519/go,polyverse-security/go,ImJasonH/go,dterei/go,4ad/go,sndnvaps/go,Cofyc/go,Endika/go,codestation/go,linux-on-ibm-z/go,codestation/go,gdey/go,jacobsa/go,4ad/go,vanloswang/go,akutz/go,gdey/go,deft-code/go,dterei/go,Samurais/go,rachtsingh/gogojuice,Endika/go,vsekhar/elastic-go,josharian/go,ImJasonH/go,cyberroadie/go,4ad/go,sndnvaps/go,rachtsingh/gogojuice,fjballest/golang,ganboing/go-esx,goodlang/good,miolini/go,cloudflare/go,cyberroadie/go,malvira/go,miolini/go,suncycheng/go,codestation/go,mwhudson/go,gdey/go,akutz/go,codestation/go,polyverse-security/go,josharian/go,cloudflare/go,jacobsa/go,nawawi/go,nawawi/go,derekmarcotte/go,mwhudson/go,mwhudson/go,cyberroadie/go,vanloswang/go,sndnvaps/go,akutz/go,malvira/go,linux-on-ibm-z/go,CAFxX/go,roth1002/go,Cofyc/go,suncycheng/go,ganboing/go-esx,theif519/go,gdey/go,binarycrusader/go,derekmarcotte/go,vsekhar/elastic-go,malvira/go,theif519/go,Endika/go,kim-racktop/go,akutz/go,ImJasonH/go,suncycheng/go,vanloswang/go,miolini/go,vanloswang/go,golang/go,gdey/go,odeke-em/go,fjballest/golang,malvira/go,Jordanzuo/go,4ad/go,goodlang/good,miolini/go,theif519/go,josharian/go,vanloswang/go,roth1002/go,suncycheng/go,golang/go,rachtsingh/gogojuice,josharian/go,deft-code/go,vsekhar/elastic-go,dterei/go,sndnvaps/go,Endika/go,fjballest/golang,ganboing/go-esx,4ad/go,cyberroadie/go,jacobsa/go,polyverse-security/go,binarycrusader/go,rachtsingh/gogojuice,momchil-velikov/go,ImJasonH/go,Endika/go,ganboing/go-esx,josharian/go,Samurais/go,CAFxX/go,theif519/go,momchil-velikov/go,codestation/go,sndnvaps/go,theif519/go,malvira/go,deft-code/go,golang/go,kim-racktop/go,Jordanzuo/go,deft-code/go,deft-code/go,sndnvaps/go,Jordanzuo/go,4ad/go,golang/go,fjballest/golang,Jordanzuo/go,vanloswang/go,kim-racktop/go,golang/go,polyverse-security/go,goodlang/good,ImJasonH/go,derekmarcotte/go,nawawi/go,vsekhar/elastic-go,rachtsingh/gogojuice,momchil-velikov/go,derekmarcotte/go,kim-racktop/go,vsekhar/elastic-go,Cofyc/go,momchil-velikov/go,binarycrusader/go,jacobsa/go,ImJasonH/go,christopher-henderson/Go,theif519/go,nawawi/go,nawawi/go,nawawi/go,jacobsa/go,kim-racktop/go,mwhudson/go,codestation/go,binarycrusader/go,ImJasonH/go,miolini/go,kim-racktop/go,linux-on-ibm-z/go,Cofyc/go,codestation/go,binarycrusader/go,codestation/go,josharian/go,sndnvaps/go,gdey/go,gdey/go,4ad/go,dterei/go,polyverse-security/go,roth1002/go,Samurais/go,vanloswang/go,Jordanzuo/go,roth1002/go,Samurais/go,cloudflare/go,gdey/go,binarycrusader/go,suncycheng/go,rachtsingh/gogojuice,derekmarcotte/go,malvira/go,dterei/go,goodlang/good,dterei/go,cloudflare/go,dterei/go,Jordanzuo/go,rachtsingh/gogojuice,cloudflare/go,goodlang/good,fjballest/golang,Jordanzuo/go,goodlang/good,rachtsingh/gogojuice,Endika/go,miolini/go,cloudflare/go,CAFxX/go,binarycrusader/go,linux-on-ibm-z/go,fjballest/golang,mwhudson/go,odeke-em/go,odeke-em/go,cyberroadie/go,theif519/go,CAFxX/go,Samurais/go,deft-code/go,Cofyc/go,derekmarcotte/go,malvira/go,jacobsa/go,roth1002/go,mwhudson/go,jacobsa/go,akutz/go,4ad/go,cloudflare/go,Samurais/go,ganboing/go-esx,mwhudson/go,goodlang/good,Cofyc/go,momchil-velikov/go,odeke-em/go,momchil-velikov/go,nawawi/go,roth1002/go,deft-code/go,binarycrusader/go,cloudflare/go,odeke-em/go,deft-code/go,CAFxX/go,roth1002/go,miolini/go,nawawi/go,Jordanzuo/go,ganboing/go-esx,linux-on-ibm-z/go,fjballest/golang,vanloswang/go,akutz/go,dterei/go,linux-on-ibm-z/go,linux-on-ibm-z/go,ganboing/go-esx,jacobsa/go,suncycheng/go,polyverse-security/go,vsekhar/elastic-go,Cofyc/go,sndnvaps/go,CAFxX/go,mwhudson/go,Samurais/go,Endika/go,miolini/go,kim-racktop/go,golang/go,Samurais/go,Endika/go,polyverse-security/go,vsekhar/elastic-go,odeke-em/go,roth1002/go,golang/go,Cofyc/go,ganboing/go-esx,polyverse-security/go,akutz/go,josharian/go,suncycheng/go,momchil-velikov/go,vsekhar/elastic-go,cyberroadie/go,kim-racktop/go,goodlang/good,cyberroadie/go,momchil-velikov/go,ImJasonH/go