Issue #294: Use upstream hostname for request

WIP WIP WIP WIP WIP WIP WIP WIP WIP WIP WIP WIP
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This patch is work in progress and demonstrates the
desired behavior and adds a test to verify it. The
feature needs to be made configurable per route.

Fabio does not modify the Host header when forwarding
the request to the upstream server. This patch enables
a mode where for a specific route fabio will use the
hostname of the upstream server as the host header.

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Fixes #294

proxy/http_integration_test.go

@@ -117,6 +117,34 @@ func TestProxyStripsPath(t *testing.T) {
 	}
 }
 
+func TestProxyUseUpstreamHostname(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, r.Host)
+	}))
+
+	proxy := httptest.NewServer(&HTTPProxy{
+		Transport: &http.Transport{
+			Dial: func(network, addr string) (net.Conn, error) {
+				addr = server.URL[len("http://"):]
+				return net.Dial(network, addr)
+			},
+		},
+		Lookup: func(r *http.Request) *route.Target {
+			tbl, _ := route.NewTable("route add mock / http://a.com/")
+			return tbl.Lookup(r, "", route.Picker["rr"], route.Matcher["prefix"])
+		},
+	})
+	defer proxy.Close()
+
+	resp, body := mustGet(proxy.URL + "/")
+	if got, want := resp.StatusCode, http.StatusOK; got != want {
+		t.Fatalf("got status %d want %d", got, want)
+	}
+	if got, want := string(body), "a.com"; got != want {
+		t.Fatalf("got body %q want %q", got, want)
+	}
+}
+
 func TestProxyLogOutput(t *testing.T) {
 	// build a format string from all log fields and one header field
 	fields := []string{"header.X-Foo:$header.X-Foo"}

proxy/http_proxy.go

@@ -99,6 +99,7 @@ func (p *HTTPProxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	} else {
 		targetURL.RawQuery = t.URL.RawQuery + "&" + r.URL.RawQuery
 	}
+	r.Host = targetURL.Host
 
 	// TODO(fs): The HasPrefix check seems redundant since the lookup function should
 	// TODO(fs): have found the target based on the prefix but there may be other