Added failing test

URL errors are raised in requests before the pre_request hook fires, so
the on_pre_request method does not run, and we get a KeyError - which is
difficult to debug - a few lines down the road in:

	request_meta["response_time"] = ...
Re-raise invalid URL exceptions separately

We're not sending any request anyway, so we need to report this.
Merge pull request #72 from krallin/enhancement/URLError

Better handling of invalid URL schemas