Merge secondary-dns-lookup-error-7408 

Author: exarkun (sponsored by Hybrid Logic, Ltd.)
Reviewer: glyph
Fixes: #7408

Fix a regression in `SecondaryAuthority` which prevented it from working in most 
basic uses.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43058 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb