Add support for Activities.ListStargazers response with timestamp.

Add custom header to request to get an alternate response with star creation timestamp.

Solves google#319
Closes google#320

github/activity_star.go

@@ -13,10 +13,16 @@ type StarredRepository struct {
 	Repository *Repository `json:"repo,omitempty"`
 }
 
+// Stargazer represents a user that has starred a repository.
+type Stargazer struct {
+	StarredAt *Timestamp `json:"starred_at,omitempty"`
+	User      *User      `json:"user,omitempty"`
+}
+
 // ListStargazers lists people who have starred the specified repo.
 //
 // GitHub API Docs: https://developer.github.com/v3/activity/starring/#list-stargazers
-func (s *ActivityService) ListStargazers(owner, repo string, opt *ListOptions) ([]User, *Response, error) {
+func (s *ActivityService) ListStargazers(owner, repo string, opt *ListOptions) ([]Stargazer, *Response, error) {
 	u := fmt.Sprintf("repos/%s/%s/stargazers", owner, repo)
 	u, err := addOptions(u, opt)
 	if err != nil {
@@ -28,7 +34,10 @@ func (s *ActivityService) ListStargazers(owner, repo string, opt *ListOptions) (
 		return nil, nil, err
 	}
 
-	stargazers := new([]User)
+	// TODO: remove custom Accept header when this API fully launches
+	req.Header.Set("Accept", mediaTypeStarringPreview)
+
+	stargazers := new([]Stargazer)
 	resp, err := s.client.Do(req, stargazers)
 	if err != nil {
 		return nil, resp, err

github/activity_star_test.go

@@ -19,19 +19,20 @@ func TestActivityService_ListStargazers(t *testing.T) {
 
 	mux.HandleFunc("/repos/o/r/stargazers", func(w http.ResponseWriter, r *http.Request) {
 		testMethod(t, r, "GET")
+		testHeader(t, r, "Accept", mediaTypeStarringPreview)
 		testFormValues(t, r, values{
 			"page": "2",
 		})
 
-		fmt.Fprint(w, `[{"id":1}]`)
+		fmt.Fprint(w, `[{"starred_at":"2002-02-10T15:30:00Z","user":{"id":1}}]`)
 	})
 
 	stargazers, _, err := client.Activity.ListStargazers("o", "r", &ListOptions{Page: 2})
 	if err != nil {
 		t.Errorf("Activity.ListStargazers returned error: %v", err)
 	}
 
-	want := []User{{ID: Int(1)}}
+	want := []Stargazer{{StarredAt: &Timestamp{time.Date(2002, time.February, 10, 15, 30, 0, 0, time.UTC)}, User: &User{ID: Int(1)}}}
 	if !reflect.DeepEqual(stargazers, want) {
 		t.Errorf("Activity.ListStargazers returned %+v, want %+v", stargazers, want)
 	}