OK, here is a patch providing both two test cases and the fix for current trunk. Will probably hack something for python 3 as well, although there the Message.as_string approach works due to the new headerlength argument defaulting to 0. So there I'd adjust the patch 

I also included a second test e-mail together with two disabled test cases in order to address the whitespace issue I mentioned. Would be nice to have a fix for these as well, but I assume you don't want known to fail test cases without a fix in trunk unless it's really serious, right?

In any case, the fact that there should be at least four tests, maybe more in the future, all dealing with signature preservation, led me to have a class dedicated to them, instead of only adding methods to one of the existing classes. I hope you agree with that decision.