[websync] Update regex to ensure note content is synchronized

Add unit tests demonstrating the problem; <note-content> elements
with unexpected attributes caused us to send empty content during
web sync.

This can happen when note XML is generated by alternative Tomboy
note clients like Gnote.

A better fix would be to stop using a regex to parse this data, but
fixing the regex seemed more appropriate for a stable bugfix release.

https://bugzilla.gnome.org/show_bug.cgi?id=632535