support allow_delete_body for protoc-gen-grpc-gateway (#318)
support allow_delete_body for protoc-gen-grpc-gateway (#318)

this fixes up #280
improbable-io/grpc-gateway,ilius/grpc-gateway,ilius/grpc-gateway,grpc-ecosystem/grpc-gateway,tmc/grpc-gateway,ilius/grpc-gateway,tmc/grpc-gateway,gengo/grpc-gateway,gengo/grpc-gateway,improbable-io/grpc-gateway