Fix how locale_on_url strips default lang.

We found a bug in how URLs are stripped of locale info when the default lang is passed.  Previously I had used the default_lang as a sentinel value that indicated no lang could be found.  The problem is, this did double duty with the user also passing in the default_lang.  This fixes things so that we return "unknown" instead of the default_lang when we can't find a match, and then bail from doing URL rewriting (i.e., we only rewrite URLs for langs we know about).  I'm not sure why I didn't think of this before.