text: Title: Flask `test_client` could not find endpoints registred from namesapces
  Body: I'm testing my flask app using pytest, when it comes to test the enpoints I used the Flask `app.test_client` utility:
  
  ```python
  @pytest.fixture(scope='session')
  def client(app):
      return app.test_client()
  ```
  Trying the test the endpoints as follow:
  
  ```python
  def test_get_all(session, client):
      sites = create_batch(SitesFactory, 4)
      # assert Sites.query.count() == 4 # returns True which means sites exist in the database
      response = client.get('/api/v1/sites')
      # tested with and without prefix does not find it both ways
      # response = client.get('/sites')
      assert response.status_code == 200
      assert len(response.json) == 4
  ```
  
  But I get always `<Response streamed [404 NOT FOUND]>` when I test it against endpoint registered from  restplus`Namespace`:
  
  ```python
  session = <sqlalchemy.orm.scoping.scoped_session object at 0x10b63eb00>
  client = <FlaskClient <Flask 'sites_service'>>
  
      def test_get_all(session, client):
          sites = create_batch(SitesFactory, 4)
          response = client.get('/api/v1/sites')
  >       assert response.status_code == 200
  E       assert 404 == 200
  E        +  where 404 = <Response streamed [404 NOT FOUND]>.status_code
  
  tests/sites/test_api.py:9: AssertionError
  
   tests/sites/test_api.py ⨯                                                                    100% ██████████
  
  Results (0.32s):
         1 failed
           - tests/sites/test_api.py:6 test_get_all
  ```
  
   but works fine if the endpoint is registered using `app.add_url_rule`. It seems like an issue (#356 ) have been also reported in the past but not solved.
  
  I'm using `pytest (3.4.1)`, `Flask (0.12.2)`, `flask-restplus (0.10.1)` on OSX environment.