Merging contentdecoderagent-test-stability-7680: Improve stability of t.w.test.ContentDecoderAgentTests.test_existingHeaders

Author: multani
Reviewer: hawkowl
Fixes: #7680

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43362 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb