Support fo POW

@kenrose @emwalker 

Made changes to allow support for POW. 

``env["REQUEST_PATH"]`` is ``nil`` when running under POW, but is not ``nil`` using nginx, WEBrick, thin, etc. This causes POW to fall into a redirect loop.

This code allows for fallback to ``env["REQUEST_URI"]`` in the case that ``env["REQUEST_PATH"]`` is ``nil``.