GMarkup: clear attributes on ignorned tags

Make sure that if we ignore a tag then we also clear the attributes that
we already collected so that they don't end up on the next unignored tag
opening.

Also add some extra brackets for clarity (it doesn't make any difference
-- I just think it reads nicer this way).

https://bugzilla.gnome.org/show_bug.cgi?id=665634
tests/gdbus-introspection: Add a less trivial test case for _IGNORE_QUALIFIED

This is distilled from PackageKit which currently aborts on startup.

https://bugzilla.gnome.org/show_bug.cgi?id=665634