properly handle API errors with no response body

this is particularly important for methods like
Organizations.CheckMembership which return a 404 to indicate a
successful response of "false".

github.go

@@ -207,12 +207,11 @@ func CheckResponse(r *http.Response) error {
 	}
 	data, err := ioutil.ReadAll(r.Body)
 	if err == nil {
-		errorResponse := new(ErrorResponse)
-		err = json.Unmarshal(data, errorResponse)
-		if err == nil && errorResponse != nil {
-			errorResponse.Response = r
-			return errorResponse
+		errorResponse := &ErrorResponse{Response: r}
+		if data != nil {
+			err = json.Unmarshal(data, errorResponse)
 		}
+		return errorResponse
 	}
 	return fmt.Errorf("github: got HTTP response code %d and error reading body: %v",
 		r.StatusCode, err)

github_test.go

@@ -159,6 +159,30 @@ func TestCheckResponse(t *testing.T) {
 	}
 }
 
+// ensure that we properly handle API errors that do not contain a response
+// body
+func TestCheckResponse_noBody(t *testing.T) {
+	res := &http.Response{
+		Request:    &http.Request{},
+		StatusCode: 400,
+		Body:       ioutil.NopCloser(strings.NewReader("")),
+	}
+	err := CheckResponse(res).(*ErrorResponse)
+
+	if err == nil {
+		t.Errorf("Expected error response.")
+	}
+
+	want := &ErrorResponse{
+		Response: res,
+		Message:  "",
+		Errors:   nil,
+	}
+	if !reflect.DeepEqual(err, want) {
+		t.Errorf("Error = %#v, want %#v", err, want)
+	}
+}
+
 func TestErrorResponse_Error(t *testing.T) {
 	res := &http.Response{Request: &http.Request{}}
 	err := ErrorResponse{Message: "m", Response: res}