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.
Merge pull request #574 from chrisguidry/fetch_favicon_fix

Prevents an error fetching favicons.