don't use http.Error with empty response bodies

internally, http.Error uses fmt.Fprintln() which will append a newline
to what should be an empty response body.  In one of my previous tests,
this was causing spurious errors.  I've since rewritten that test, but
still best to avoid it here.

github/orgs_test.go

@@ -189,12 +189,12 @@ func TestOrganizationsService_CheckMembership_notMember(t *testing.T) {
 		if r.Method != "GET" {
 			t.Errorf("Request method = %v, want %v", r.Method, "GET")
 		}
-		http.Error(w, "", http.StatusNotFound)
+		w.WriteHeader(http.StatusNotFound)
 	})
 
 	member, err := client.Organizations.CheckMembership("o", "u")
 	if err != nil {
-		t.Errorf("Organizations.CheckMembership returned error: %v", err)
+		t.Errorf("Organizations.CheckMembership returned error: %+v", err)
 	}
 	want := false
 	if member != want {
@@ -254,7 +254,7 @@ func TestOrganizationsService_CheckPublicMembership_notMember(t *testing.T) {
 		if r.Method != "GET" {
 			t.Errorf("Request method = %v, want %v", r.Method, "GET")
 		}
-		http.Error(w, "", http.StatusNotFound)
+		w.WriteHeader(http.StatusNotFound)
 	})
 
 	member, err := client.Organizations.CheckPublicMembership("o", "u")