Fix assert-is-always-True in twisted tests

[Callek]

This issue was found during my work on #58 (and is where I added myself to authors)

I don't feel this is worth adding to the changelog, and is the only place (other than #58) in which this was a problem afaict.

[Callek]

...looks like this is failing after fixing the assert, I don't admit to knowing enough of twisted to get this test right, without simultaneously worrying that I'll also break the thing its trying to test though...

[hynek]

Yeah test suite fixes don't need a change log entry.

The tests seem to fail now tho. :)

[hynek]

I guess that Failures aren't comparable. Will look at it when I'm back. Will probably need to split up the assert into an isinstance and comparing .value