title: mocked service call is not being used
body: <p>I have the model object that looks like this</p>
  <pre><code>class CatalogModel(BaseModel):
      @property
      def custom_service(self):
          return CustomService()
  
      async def get_offers(self, catalog_name):
          try:
              svc_response = self.custom_service.get_offers(catalog_name=catalog_name)()
          except BaseException:
              raise SalesForceException()
          return CustomOfferResponse().dump(svc_response)
  </code></pre>
  <p>I am trying to write a test for that <code>get_offers</code> function (that uses <code>custom_services</code> which is connecting to Salesforce)</p>
  <p>My test is looking like this. I am using <code>pytest</code>, <code>pytest vcr</code> etc.</p>
  <pre><code>class TestCatalogModel:
      catalog_name = 'CATALOG_1'
      
      @freeze_time(&quot;2021-07-12&quot;)
      async def test_get_offers(self, loop, offers, offers_response):
          with MockUser(ident=&quot;test_model_get_offers&quot;):
              with patch(
                  &quot;com.services.client.CustomService.get_offers&quot;, new=offers
              ):
                  eo_offers = await CatalogModel().get_offers(self.catalog_name)
                  assert offers_response == eo_offers
  </code></pre>
  <p>However when executing the test it fails with the error</p>
  <pre><code>E   vcr.errors.CannotOverwriteExistingCassetteException: Can't overwrite existing cassette ('/test_api/recordings/2021-07-12/test_get_offers_model/salesforce/auth/client/services_oauth2_token.yaml') in your current record mode ('none').
  E   No match for the request (&amp;lt;Request (POST) https://server.salesforce.com/services/oauth2/token&amp;gt;) was found.
  E   No similar requests, that have not been played, found.
  
  During handling of the above exception, another exception occurred:
  ...model.py:17: in test_get_offers
      eo_offers = await CatalogModel().get_offers(self.catalog_name)
  model.py:24: in get_offers
      raise SalesForceException()
  E   ...SalesForceException: Error from Salesforce.
  </code></pre>
  <p>As far as I understand it is trying to connect to real Salesforce service, rather than using a mock. What is the problem?</p>