Respond with correct content-type header

[Bathtor]

I updated grpc_server to always insert the content-type used by the request (or application/grpc by default) into the response's HTTP headers.

Without this, I couldn't get it to communicate with https://github.com/grpc/grpc-java, since the latter just rejected every message with undefined content-type headers and setting them manually for every single response seemed very inefficient.

[WangScaler]

It's right ！