RuntimeWarning about closing a session in an async mode

I'm getting this warning when using session in an async mode:
```
/home/seer/.local/share/virtualenvs/pbJ32E1R/lib/python3.7/site-packages/jsonapi_client/session.py:288: RuntimeWarning: coroutine 'ClientSession.close' was never awaited
  self._aiohttp_session.close()
```
As it says, `close()` coroutine should be awaited somewhere to the session properly.