get the decorated function in a way that is compatible with 2.6

while we're at it, be consistent with the other spelling of "wrappee" in
this module

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/deprecated-instance-methods-4804-2@43555 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb