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.
Do we need to check newText and oldText for being null?

I don't know how that method is used, but I'm wondering what happens if the text changes but has the same length, i.e. replacing 'test' with 'text'.