add POST params to fetch_request_token in oauth1_session

[kasajei]

Some service need POST params for the method.

[coveralls]

Coverage remained the same when pulling 8045b21 on kasajei:master into 08727f0 on requests:master.

[ib-lundgren]

Please rename to request_kwargs and a param line in the doc below

[ib-lundgren]

Good idea :) See inline comments for a few nitpicks.

[coveralls]

Coverage remained the same when pulling c5c7bf3 on kasajei:master into 08727f0 on requests:master.

[kasajei]

@ib-lundgren Thank you for you comment.
I would not add kwargs but data for request, so refactored the parameter name.

[ib-lundgren]

Ah I see. Still I think having the ability to add any keyword arg you may need would be useful and something I've been meaning to add for a while. Also, while data is currently a second position arg it is primarily a keyword arg and ideally we want to avoid depending on position.

Changing to

 oauth_sess.post(url, request_kwargs={'data': data})

is slightly more inconvenient than

 oauth_sess.post(url, request_data=data)

but allow users to use all of requests key words without us having to add them explicitly. For example changing headers and stream.

 headers = {'Accept': 'application/json'}
 oauth_sess.post(url, request_kwargs={'data': data, 'headers':headers, 'stream': True})

[Lukasa]

Closed in favour of #226.