fix when multiple actions have the same url_path

[shipengtaov]

this pull request fix case when multiple actions having the same url_path, while not using action.mapping

@action(detail=False, methods=['get'], url_path='test')
def action_get(self, request):
    pass

@action(detail=False, methods=['post'], url_path='test')
def action_post(self, request):
    pass

In this case, the swagger will only show GET /test, not including POST /test