Moved decision of whether to sign post fields to a function.

Some non-conformant APIs that have incorrect implementations of Oauth may require this method to be overridden.
Changed tab to space.
Merge pull request #291 from Danack/upstream

Moved decision of whether to sign post fields to a function.
Merge branch 'upstream'

Conflicts:
	src/Guzzle/Plugin/Oauth/OauthPlugin.php
Removed white space changes.