Mention that Do/Receive won't decode for status code 204 (#56)

* Improve the godoc for `Do` and add no content tests

sling.go

@@ -359,6 +359,7 @@ func (s *Sling) ReceiveSuccess(successV interface{}) (*http.Response, error) {
 // Receive creates a new HTTP request and returns the response. Success
 // responses (2XX) are JSON decoded into the value pointed to by successV and
 // other responses are JSON decoded into the value pointed to by failureV.
+// If the status code of response is 204(no content), decoding is skipped.
 // Any error creating the request, sending it, or decoding the response is
 // returned.
 // Receive is shorthand for calling Request and Do.
@@ -373,6 +374,7 @@ func (s *Sling) Receive(successV, failureV interface{}) (*http.Response, error)
 // Do sends an HTTP request and returns the response. Success responses (2XX)
 // are JSON decoded into the value pointed to by successV and other responses
 // are JSON decoded into the value pointed to by failureV.
+// If the status code of response is 204(no content), decoding is skipped.
 // Any error sending the request or decoding the response is returned.
 func (s *Sling) Do(req *http.Request, successV, failureV interface{}) (*http.Response, error) {
 	resp, err := s.httpClient.Do(req)
@@ -383,7 +385,7 @@ func (s *Sling) Do(req *http.Request, successV, failureV interface{}) (*http.Res
 	defer resp.Body.Close()
 
 	// Don't try to decode on 204s
-	if resp.StatusCode == 204 {
+	if resp.StatusCode == http.StatusNoContent {
 		return resp, nil
 	}
 

sling_test.go

@@ -665,6 +665,36 @@ func TestDo_onSuccessWithNilValue(t *testing.T) {
 	}
 }
 
+func TestDo_noContent(t *testing.T) {
+	client, mux, server := testServer()
+	defer server.Close()
+	mux.HandleFunc("/nocontent", func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(204)
+	})
+
+	sling := New().Client(client)
+	req, _ := http.NewRequest("DELETE", "http://example.com/nocontent", nil)
+
+	model := new(FakeModel)
+	apiError := new(APIError)
+	resp, err := sling.Do(req, model, apiError)
+
+	if err != nil {
+		t.Errorf("expected nil, got %v", err)
+	}
+	if resp.StatusCode != 204 {
+		t.Errorf("expected %d, got %d", 204, resp.StatusCode)
+	}
+	expectedModel := &FakeModel{}
+	if !reflect.DeepEqual(expectedModel, model) {
+		t.Errorf("successV should not be populated, exepcted %v, got %v", expectedModel, model)
+	}
+	expectedAPIError := &APIError{}
+	if !reflect.DeepEqual(expectedAPIError, apiError) {
+		t.Errorf("failureV should not be populated, exepcted %v, got %v", expectedAPIError, apiError)
+	}
+}
+
 func TestDo_onFailure(t *testing.T) {
 	const expectedMessage = "Invalid argument"
 	const expectedCode int = 215
@@ -830,6 +860,25 @@ func TestReceive_failure(t *testing.T) {
 	}
 }
 
+func TestReceive_noContent(t *testing.T) {
+	client, mux, server := testServer()
+	defer server.Close()
+	mux.HandleFunc("/foo/submit", func(w http.ResponseWriter, r *http.Request) {
+		assertMethod(t, "HEAD", r)
+		w.WriteHeader(204)
+	})
+
+	endpoint := New().Client(client).Base("http://example.com/").Path("foo/").Head("submit")
+	resp, err := endpoint.New().Receive(nil, nil)
+
+	if err != nil {
+		t.Errorf("expected nil, got %v", err)
+	}
+	if resp.StatusCode != 204 {
+		t.Errorf("expected %d, got %d", 204, resp.StatusCode)
+	}
+}
+
 func TestReceive_errorCreatingRequest(t *testing.T) {
 	expectedErr := errors.New("json: unsupported value: +Inf")
 	resp, err := New().BodyJSON(FakeModel{Temperature: math.Inf(1)}).Receive(nil, nil)