Fix tests to adapt to the changes in 95edaa5d87c3c654061edea1896cd1d44d10d99e (specifically, that when autodiscovery is enabled FeedClient.fetch passes feed.url to RestClient.get, instead of feed.fetch_url).
When saving a feed, set default values to attributes only after fixing any encoding problems in title, url and fetch_url attributes.

This is necessary since commit a49d70e150f7797de4ef2b0e127da17c3b11bec9, because the url attribute defaults to the fetch_url value. If fetch_url has encoding problems and we attempt to copy fetch_url to url before fixing them, an error will be raised. The fix is just to fix encoding, then set default attribute values.
Fix tests. Complements 47758f330540a4464d2f952ad98e97557be08a13.