fix TestCompressOK and client reserved HTTP header handling

server.go

@@ -460,6 +460,10 @@ func (s *Server) processUnaryRPC(t transport.ServerTransport, stream *transport.
 			}
 		}()
 	}
+	if s.opts.cp != nil {
+		// NOTE: this needs to be ahead of all handling, https://github.com/grpc/grpc-go/issues/686.
+		stream.SetSendCompress(s.opts.cp.Type())
+	}
 	p := &parser{r: stream}
 	for {
 		pf, req, err := p.recvMsg()
@@ -545,9 +549,6 @@ func (s *Server) processUnaryRPC(t transport.ServerTransport, stream *transport.
 			Last:  true,
 			Delay: false,
 		}
-		if s.opts.cp != nil {
-			stream.SetSendCompress(s.opts.cp.Type())
-		}
 		if err := s.sendResponse(t, stream, reply, s.opts.cp, opts); err != nil {
 			switch err := err.(type) {
 			case transport.ConnectionError: