Prevents an error fetching favicons.

When encountering a favicon `<link>` where the `href` is not enclosed
in quotes, the selfoss `ContentLoader` would fail and exit early.

For example, `<link rel="shortcut icon" href=img/favicon.ico>` would cause
the error `Undefined offset 2` in `helpers/Image.php`.  Making the regex
a bit more liberal and also checking the results of the final `preg_match`
fix this.

This fixes the issue I asked about in http://selfoss.aditu.de/forum/index.php?id=936.
This is also the corrected followup to the incorrectly-based PR in https://github.com/SSilence/selfoss/pull/572.