Make redirect work with Proxy

[jeckman]

Currently if the user is not in "download" mode (where they pick a format from the index.php list), they get redirected to the best format based on the request.

However, this only works with the redirect directly to source url - it bypasses the proxy option.

It would be better if direct access could still leverage proxy if the proxy configuration was set - maybe "proxy" should be sent in the query string (where the enclosure url is in the feed.xml file).