AssertionError: Task exception was never retrieved when running tests

[leszekhanusz]

Sometimes when running the tests, the following warning appears:

ERROR:asyncio:Task exception was never retrieved
future: <Task finished name='Task-488' coro=<WebsocketsTransport._receive_data_loop() done, defined at /mnt/tera/git-repos/leszekhanusz/gql/gql/transport/websockets.py:333> exception=AssertionError()>
Traceback (most recent call last):
  File "/mnt/tera/git-repos/leszekhanusz/gql/gql/transport/websockets.py", line 340, in _receive_data_loop
    answer = await self._receive()
  File "/mnt/tera/git-repos/leszekhanusz/gql/gql/transport/websockets.py", line 169, in _receive
    assert self.websocket is not None
AssertionError

Even though the tests are passing.
It is probably harmless but we should investigate why it happens.

[leszekhanusz]

Should be fixed by #228