Fix incorrect response handling in redirect check

The activation of the curl option NOBODY, in 0c019b44, results in curl making HEAD requests instead of GET requests.
Sadly a HEAD request on the news feed from arxiv.org is answered by a '405 Method Not Allowed', which results in the redirect checks to fail, and keeps the feeds from being updated.

This patch considers a 405 as a valid response, and returns the currently considered url as final.