Fix bug where Twiggy's run loop exits after a streaming request is cut off

Hello,

I found a bug in Twiggy where if you're using a streaming writer and the client stops the request, the PSGI application dies in such a way that Twiggy itself shuts down.  A test file and fix are included.  The fix isn't the cleanest, but maybe it's a starting point you could improve upon!

-Rob