Fixed an issue where the key of headers becomes always lower case, etc.

Hi. Thank you for the awesome library.

I found a bug.
However, I don't know whether right or wrong.

Is this patch all right?

----
Fixed an issue where the key of headers becomes always lower case.
Fixed an error that occurs when the request is posted without specifying the data argument.

Following error message

'NoneType' object has no attribute 'items'
lib/python2.7/site-packages/rauth-0.5.5-py2.7.egg/rauth/oauth.py in _normalize_request_parameters, line 77

cause

The default argument of the post method of the requests module is None.
----

Thanks.