Fix unannonced trailers problem when body is empty

Gopkg.lock

@@ -1350,7 +1350,7 @@
     "roundrobin",
     "utils"
   ]
-  revision = "7d94d212f808222b72fd0b8bb171bfcd4e27ffca"
+  revision = "c34b0c501e43223bc816ac9b40b0ac29c44c8952"
 
 [[projects]]
   name = "github.com/vulcand/predicate"

vendor/github.com/vulcand/oxy/forward/fwd.go

@@ -548,6 +548,16 @@ func (f *httpForwarder) serveHTTP(w http.ResponseWriter, inReq *http.Request, ct
 	} else {
 		revproxy.ServeHTTP(w, outReq)
 	}
+
+	for key := range w.Header() {
+		if strings.HasPrefix(key, http.TrailerPrefix) {
+			if fl, ok := w.(http.Flusher); ok {
+				fl.Flush()
+			}
+			break
+		}
+	}
+
 }
 
 // IsWebsocketRequest determines if the specified HTTP request is a