Distinguish redirects from non-redirects by Location header, not just status code

[JesseWeinstein]

I've seen quite a few shortners that seem to return 301 for both existing AND non-existing shortcodes.

I suggest that an additional project setting be added, called location_regex, which if non-empty, would be used after the redirect status code as a way to handle such shortners. If the location_regex matched the Location: header, the short code would be considered a non-redirect, even though it had a redirect status code.

[chfoo]

Closing because it was implemented and merged in the pull request already