2018-04-18 Version 0.4.28

Features

msrest is now able to keep the "requests.Session" alive for performance. To activate this behavior:
- Use the final Client as a context manager (requires generation with Autorest.Python 3.0.50 at least)
- Use client.config.keep_alive = True and client.close() (requires generation with Autorest.Python 3.0.50 at least)
- Use client.config.keep_alive = True and client._client.close() (not recommended, but available in old releases of SDK)
All Authentication classes now define signed_session and refresh_session with an optional session parameter.
To take benefits of the session improvement, a subclass of Authentication MUST add this optional parameter
and use it if it's not None:
```
   def signed_session(self, session=None):
       session = session or requests.Session()

       # As usual from here.
```