Fix error responses not including a content-length
Fix error responses not including a content-length

This would prevent the client from knowing whether the request
was received fully without the connection closing first.

CrankWheel/cowboy,kivra/cowboy,hairyhum/cowboy,turtleDeng/cowboy,K2InformaticsGmbH/cowboy,bsmr-erlang/cowboy,ninenines/cowboy,rabbitmq/cowboy