Make twisted.web.test.test_http.ParsingTestCase.testMissingContentDisposition pass on Python 3

[exarkun]

Presently this test is skipped on Python 3 because it will fail or go into an infinite loop. cgi.parse_multipart is currently  broken on Python 3. We may need to implement our own multipart/form-data parser (might be a good opportunity to use  Parsley?). Or we could try to fix the stdlib code and then ship a backport (or depend on Python 3.4 instead of Python 3.3).

Note also our test coverage for handling of multipart/form-data is really awful - #6111.

[itamar]

 web2.fileupload and  corresponding tests should at the very least provide a basis (and some useful tests) for what an implementation needs to do. Also a reference for what browser stupidity needs working around.