Apply adiroiban's 1333-1.diff.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/ftp-list-flags-1333@37408 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb
Prefer assertIn.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/ftp-list-flags-1333@37409 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb