Flush when we have unannonced trailers

.travis.yml

@@ -1,7 +1,6 @@
 language: go
 
 go:
-  - 1.8.x
   - 1.9.x
   - 1.10.x
   - 1.11.x

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

forward/fwd_test.go

@@ -3,6 +3,7 @@ package forward
 import (
 	"context"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
@@ -409,3 +410,25 @@ func TestTeTrailer(t *testing.T) {
 	assert.Equal(t, http.StatusOK, re.StatusCode)
 	assert.Equal(t, "trailers", teHeader)
 }
+
+func TestUnannouncedTrailer(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		rw.WriteHeader(200)
+		rw.(http.Flusher).Flush()
+
+		rw.Header().Add(http.TrailerPrefix+"X-Trailer", "foo")
+	}))
+
+	proxy, err := New()
+	require.Nil(t, err)
+
+	proxySrv := httptest.NewServer(http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		req.URL = testutils.ParseURI(srv.URL)
+		proxy.ServeHTTP(rw, req)
+	}))
+
+	resp, _ := http.Get(proxySrv.URL)
+	ioutil.ReadAll(resp.Body)
+
+	require.Equal(t, resp.Trailer.Get("X-Trailer"), "foo")
+}