proxy: set Content-Type for gRPC requests

Prior we would not set the Content-Type header for
grpc request, which on failure events would cause
the response to be invalid gRPC responses, as the
grpc code path in sendDirectResponse expects the
Content-Type header to be set.

This change fixes the problem by setting the
Content-Type header to application/grpc for gRPC
requests.

proxy/proxy.go

@@ -134,6 +134,12 @@ func (p *Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// If the request is a gRPC request, we need to set the Content-Type
+	// header to application/grpc.
+	if strings.HasPrefix(r.Header.Get(hdrContentType), hdrTypeGrpc) {
+		w.Header().Set(hdrContentType, hdrTypeGrpc)
+	}
+
 	// Requests that can't be matched to a service backend will be
 	// dispatched to the static file server. If the file exists in the
 	// static file folder it will be served, otherwise the static server