Compatibility with Requests AuthBase

[aparamon]

Currently, asks supports extensible authz/authn:
https://asks.readthedocs.io/en/latest/overview-of-funcs-and-args.html#authing
https://github.com/theelous3/asks/blob/master/asks/auth.py
It is possible to inherit from AuthBase to introduce missing auth methods.

Unfortunately, asks.auth.AuthBase is not compatible with requests.auth.AuthBase:
http://docs.python-requests.org/en/master/user/authentication/#new-forms-of-authentication

It would be awesome if existing non-trivial requests auth plugins could be used with asks as well.
For example:
https://github.com/lepture/authlib/blob/dee7f003b74252e22d62e11a91ec1b629b6fdeae/authlib/client/oauth2_session.py#L22-L34

[andersea]

Specifically for signing requests to cryptocurrency exchange apis I need the body to be available in the request object.

Edit:

The body is generated here, but isn't added as an attribute of the request object.

    # handle building the request body, if any
    body = ''
    if any((self.data, self.files, self.json is not None)):
        content_type, content_len, body = await self._formulate_body()
        asks_headers['Content-Type'] = content_type
        asks_headers['Content-Length'] = content_len

See - https://www.bitmex.com/app/apiKeysUsage

Here is a requests based auth implementation:

https://github.com/BitMEX/sample-market-maker/blob/master/market_maker/auth/APIKeyAuthWithExpires.py

[andersea]

PR #128 adds the body attribute to allow the described use case.