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.
There is a comment and different values for the Assertions that suggests that something different is being done here (from the previous set of assertions) but it looks like the values passed to AdjustSpansForTextChange are identical.