Fix for updating <span> boundaries during user editing

This partially (and alas imperfectly) implements the third stage
described in http://jira.palaso.org/issues/browse/WS-34799.  Of course, perfection is probably unobtainable.
It would be helpful to add a comment that shows that what we're testing here is basically the string "This is a <strong>test</strong> of some<weak>thing or </weak>other." and maybe mention http://jira.palaso.org/issues/browse/WS-34799.