Trim feed urls to remove accidental spaces

When creating a Feed using the FeedHTTPFetcher, a feed url with a space at the end is considered invalid. Technically yes this is an invalid url. However, it's be nice if Feeds just trimmed the url to show some mercy for people copy/pasting the url and accidentally grabbing an extra space at the end.

Will write a patch soon if no one beats me too it... pre-launch week is a fun time. :)