Fix wrong header sent in echo_post example
Fix wrong header sent in echo_post example

callsign-core/cowboy_hybi00,cad627/cowboy,noname007/cowboy,nviennot/cowboy,dangjun625/cowboy,bsmr-erlang/cowboy,linearregression/cowboyku,layerhq/cowboy,ferd/cowboyku,K2InformaticsGmbH/cowboy,kivra/cowboy,manastech/cowboy,rabbitmq/cowboy,lpgauth/cowboy,essen/cowboy,grahamrhay/cowboy,omusico/cowboy,venkateshreddy/cowboy,acrispin/cowboy,manastech/cowboy,ninenines/cowboy,hairyhum/cowboy,paulperegud/cowboy,callsign-core/cowboy_hybi00,pap/cowboy,sile/cowboy,binarin/cowboy,cad627/cowboy,heroku/cowboyku,turtleDeng/cowboy,odo/cowboy,layerhq/cowboy,CrankWheel/cowboy,binarin/cowboy,essen/cowboy,imdeps/cowboy,yangchengjian/cowboy,bobos/ecweb,unisontech/cowboy,yangchengjian/cowboy,MSch/cowboy,unisontech/cowboy,lpgauth/cowboy,276361270/cowboy,Reimerei/cowboy