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.
I personally think it would be better to split this into multiple tests, one for each of the different AdjustSpansForTextChange cases. Having it all in one big test has IMO several disadvantages: 
- it's hard to see if there are any dependencies/interactions between the test cases
- if one case fails all other cases don't get executed, so it's hard to know if they would fail or pass

Splitting the test could be done
- by using NUnit's TestCaseAttribute,
- by adding a new test class that has lines 240+241 in a setup method,
- or by simply adding multiple tests that have one test case each.

(I see Jason already made a similar comment above...)