title: Mock patch method and an attribute
body: <p>I would like to patch an attribute of the data returned by a method.</p>
  
  <p>Assuming I have the following simplified piece code:</p>
  
  <pre><code>@patch('requests.post')
  class TestKeywordsApi(BaseTest):
      # Instantiate API class and set the apikey
      def setUp(self):
          BaseTest.setUp(self)
          self.fixtures = FIXTURES
          self.api = BaseApi()
  
      def mock_requests_post(self, url, data=None):
          ''' Mock method for post method from responses library.
              It replaces the responses.post calls in Api class.
          '''
          url = self.encode_url(url, data)
          if url:
              return self.fixtures[url]
  
      def test_save_success(self, mock_post):
          mock_post.side_effect = self.mock_requests_post
  
          response = self.api.post(keyword, params={...})
  
          # list of asserts
  
  # original class calling requests.post    
  import requests
  class BaseApi(object):
      def post(self, action, params):
          ''' Sends a POST request to API '''
          response = requests.post(self.build_url(action), data=params).content
  </code></pre>
  
  <p>The code above fails because the mock method does not provide a mock/stub for 'content' attribute present in requests library. Does anyone know how to stub the content attribute?</p>