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.
Checking for null might be a good idea, but I don't think it will happen in practice.  (famous last words...)

If the text changes without the length changing, there's not enough information to know how to adjust the spans, so we decided to leave them alone.  The WeSay user doesn't have a clue that there are spans underneath, but we don't want to gratuitously lose them if any edits made don't really affect them.  If we end up with meaningless spans occasionally, that's still better than consistently losing spans if the entry is edited (the status quo).