proxy: add Host header checking

fix issue #993

Signed-off-by: Tw <tw19881113@gmail.com>

caddyhttp/proxy/proxy_test.go

@@ -357,9 +357,11 @@ func TestUpstreamHeadersUpdate(t *testing.T) {
 	defer log.SetOutput(os.Stderr)
 
 	var actualHeaders http.Header
+	var actualHost string
 	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte("Hello, client"))
 		actualHeaders = r.Header
+		actualHost = r.Host
 	}))
 	defer backend.Close()
 
@@ -371,6 +373,7 @@ func TestUpstreamHeadersUpdate(t *testing.T) {
 		"+Add-Me":    {"Add-Value"},
 		"-Remove-Me": {""},
 		"Replace-Me": {"{hostname}"},
+		"Host":       {"{>Host}"},
 	}
 	// set up proxy
 	p := &Proxy{
@@ -385,10 +388,12 @@ func TestUpstreamHeadersUpdate(t *testing.T) {
 	}
 	w := httptest.NewRecorder()
 
+	const expectHost = "example.com"
 	//add initial headers
 	r.Header.Add("Merge-Me", "Initial")
 	r.Header.Add("Remove-Me", "Remove-Value")
 	r.Header.Add("Replace-Me", "Replace-Value")
+	r.Header.Add("Host", expectHost)
 
 	p.ServeHTTP(w, r)
 
@@ -421,6 +426,10 @@ func TestUpstreamHeadersUpdate(t *testing.T) {
 		t.Errorf("Request sent to upstream backend should replace value of %v header with %v. Instead value was %v", headerKey, headerValue, value)
 	}
 
+	if actualHost != expectHost {
+		t.Errorf("Request sent to upstream backend should have value of Host with %s, but got %s", expectHost, actualHost)
+	}
+
 }
 
 func TestDownstreamHeadersUpdate(t *testing.T) {