Propagate Host header from gateway request to the function.

Host HTTP header was not propagated to the function because it is not
a part of http.Request.Header map.

Signed-off-by: Dmitri Rubinstein <dmitri.rubinstein@googlemail.com>

gateway/handlers/forwarding_proxy.go

@@ -53,6 +53,9 @@ func buildUpstreamRequest(r *http.Request, url string) *http.Request {
 	}
 
 	upstreamReq, _ := http.NewRequest(r.Method, url, nil)
+	if len(r.Host) > 0 {
+		upstreamReq.Host = r.Host
+	}
 	copyHeaders(upstreamReq.Header, &r.Header)
 
 	upstreamReq.Header["X-Forwarded-For"] = []string{r.RemoteAddr}