Support for a path in marathon location (#210)

In order to join multiple marathons (e.g. per dev-team, per project) 
we are discovering their (semi-random) locations through linkerd.
This changes our --marathon-location value from the standard 
hostname:port into something like 
--marathon-location=localhost:4141/ops-staging/marathon-devteam-1
This fix checks, naively, for the existence of "path" and divides it into 
host and url that is then prefixing the path param.

Author: strzelecki-maciek

marathon/marathon.go

@@ -212,13 +212,28 @@ func (m Marathon) url(path string) string {
 
 type params map[string][]string
 
+// urlWithQuery returns absolute path to marathon endpoint
+// if location is given with path e.g. "localhost:8080/proxy/url", then
+// host and path parts are appended to respective url.URL fields
 func (m Marathon) urlWithQuery(path string, params params) string {
-	marathon := url.URL{
-		Scheme: m.Protocol,
-		User:   m.Auth,
-		Host:   m.Location,
-		Path:   path,
+	var marathon url.URL
+	if strings.Contains(m.Location, "/") {
+		parts := strings.SplitN(m.Location, "/", 2)
+		marathon = url.URL{
+			Scheme: m.Protocol,
+			User:   m.Auth,
+			Host:   parts[0],
+			Path:   "/" + parts[1] + path,
+		}
+	} else {
+		marathon = url.URL{
+			Scheme: m.Protocol,
+			User:   m.Auth,
+			Host:   m.Location,
+			Path:   path,
+		}
 	}
+
 	query := marathon.Query()
 	for key, values := range params {
 		for _, value := range values {

marathon/marathon_test.go

@@ -408,6 +408,30 @@ func TestEventStream_PassingStreamerCreated(t *testing.T) {
 	assert.IsType(t, &Streamer{}, streamer)
 }
 
+func TestUrlWithQuery_NoProxyMarathon(t *testing.T) {
+	t.Parallel()
+
+	// given
+	m, _ := New(Config{Location: "localhost:8080", Protocol: "HTTP"}, "")
+	// when
+	path := m.urlWithQuery("/testpath", params{})
+
+	// then
+	assert.Equal(t, "HTTP://localhost:8080/testpath", path)
+}
+
+func TestUrlWithQuery_ProxyMarathon(t *testing.T) {
+	t.Parallel()
+
+	// given
+	m, _ := New(Config{Location: "localhost:8080/proxy/url/segments", Protocol: "HTTP"}, "")
+	// when
+	path := m.urlWithQuery("/testpath", params{})
+
+	// then
+	assert.Equal(t, "HTTP://localhost:8080/proxy/url/segments/testpath", path)
+}
+
 // http://keighl.com/post/mocking-http-responses-in-golang/
 func stubServer(uri string, body string) (*httptest.Server, *http.Transport) {
 	return mockServer(func(w http.ResponseWriter, r *http.Request) {