Add missing ListOptions to ListLabels (#337)

Fixes : #336

examples/labels.go

@@ -21,7 +21,7 @@ func labelExample() {
 	log.Printf("Created label: %s\nWith color: %s\n", label.Name, label.Color)
 
 	// List all labels
-	labels, _, err := git.Labels.ListLabels("myname/myproject")
+	labels, _, err := git.Labels.ListLabels("myname/myproject", nil)
 	if err != nil {
 		log.Fatal(err)
 	}

labels.go

@@ -48,17 +48,22 @@ func (l Label) String() string {
 	return Stringify(l)
 }
 
+// ListLabelsOptions represents the available ListLabels() options.
+//
+// GitLab API docs: https://docs.gitlab.com/ce/api/labels.html#list-labels
+type ListLabelsOptions ListOptions
+
 // ListLabels gets all labels for given project.
 //
 // GitLab API docs: https://docs.gitlab.com/ce/api/labels.html#list-labels
-func (s *LabelsService) ListLabels(pid interface{}, options ...OptionFunc) ([]*Label, *Response, error) {
+func (s *LabelsService) ListLabels(pid interface{}, opt *ListLabelsOptions, options ...OptionFunc) ([]*Label, *Response, error) {
 	project, err := parseID(pid)
 	if err != nil {
 		return nil, nil, err
 	}
 	u := fmt.Sprintf("projects/%s/labels", url.QueryEscape(project))
 
-	req, err := s.client.NewRequest("GET", u, nil, options)
+	req, err := s.client.NewRequest("GET", u, opt, options)
 	if err != nil {
 		return nil, nil, err
 	}