Update requests_oauthlib/core.py

URL is turned into a unicode object by the signing, too. Leaving it like that will cause UnicodeDecodeError during the httplib2 request call.
we have the url and all headers to make sure none of them is of type unicode
reversing earlier changes
using new `decode` client parameter
updating oauthlib requirement
`decoding` not `decode` (certainly helps to read the source and test it afterwards!)
Merge pull request #17 from shezi/patch-1

Update requests_oauthlib/core.py