[Reviewer: Mike] Handle 6xx responses by cancelling outstanding requests

Mike,

Please can you review my fix to handle 6xx responses by cancelling outstanding requests?  This fixes #757.

I've UT-ed this, but one thing I wanted to check with you - the old code prioritized 487 responses over all other responses.  I can't see anything in RFC 3261 that says it should do.  Do you know why the old code did that?

Matt