Trying to modify url by overriding FeedsHTTPFetcher

I want to be able to modify the url before sending requests as well. I'm having some weird problem when extending FeedsHTTPFetcher and overriding the fetch() method. Even if I override the method fetch() with an exact copy in the extended class, I get Download of failed with code -1002 on import. I've read the documentation but still can't figure this out. Any ideas of what I might be missing?