add pagination support to Activity.ListWatchers

this is a breaking change, but there's no way around that, since the opt
param was left off initially.

github/activity_watching.go

@@ -15,8 +15,12 @@ type RepositorySubscription struct {
 // ListWatchers lists watchers of a particular repo.
 //
 // GitHub API Docs: http://developer.github.com/v3/activity/watching/#list-watchers
-func (s *ActivityService) ListWatchers(owner, repo string) ([]User, *Response, error) {
+func (s *ActivityService) ListWatchers(owner, repo string, opt *ListOptions) ([]User, *Response, error) {
 	u := fmt.Sprintf("repos/%s/%s/subscribers", owner, repo)
+	u, err := addOptions(u, opt)
+	if err != nil {
+		return nil, nil, err
+	}
 
 	req, err := s.client.NewRequest("GET", u, nil)
 	if err != nil {

github/activity_watching_test.go

@@ -14,10 +14,14 @@ func TestActivityService_ListWatchers(t *testing.T) {
 
 	mux.HandleFunc("/repos/o/r/subscribers", func(w http.ResponseWriter, r *http.Request) {
 		testMethod(t, r, "GET")
+		testFormValues(t, r, values{
+			"page": "2",
+		})
+
 		fmt.Fprint(w, `[{"id":1}]`)
 	})
 
-	watchers, _, err := client.Activity.ListWatchers("o", "r")
+	watchers, _, err := client.Activity.ListWatchers("o", "r", &ListOptions{Page: 2})
 	if err != nil {
 		t.Errorf("Activity.ListWatchers returned error: %v", err)
 	}