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.
With the creation of the spans being done in the method and the assertions being done with direct values it is really hard to comprehend and verify that these tests make sense. If ResetSpans was changed to SetTestSpans or CreateTestSpans and it provided the index and length of the spans and the Assertions were handled in a VerifySpans method which took the expected end result the tests would be a lot shorter, and a reader would have at least a fighting chance of understanding what was being tested.