Fix issue with `around_http_request` when using multiple threads.

Fibers cannot be shared across threads, and you would get an error
when using this feature with threads:

FiberError: fiber called across threads

Actually, we were rescuing FiberError and transforming it into an even more confusing error:

Your around_http_request hook declared at <location> must call #proceed on the yielded request but did not.

Note that VCR is not thread safe (see #200) but this brings it a bit closer.

Fixes #257.