Improve check for songs equality
Improve check for songs equality

Don't fail if one of objects is not a Song instance.

fakelbst/Chrome-Last.fm-Scrobbler,david-sabata/web-scrobbler,alexesprit/web-scrobbler,inverse/web-scrobbler,galeksandrp/web-scrobbler,carpet-berlin/web-scrobbler,carpet-berlin/web-scrobbler,david-sabata/web-scrobbler,fakelbst/Chrome-Last.fm-Scrobbler,galeksandrp/web-scrobbler,inverse/web-scrobbler,alexesprit/web-scrobbler