Bug fix and unit test for edge case failure in findText() when using a r...

The new unit test throws an exception without the change to ```rangy-textrange.js``` and I traced it back to the ```handleMatch()``` call on line 1564 being made when ```matchStartIndex``` is undefined.

If my understanding of the code is correct, we should be setting ```matchStartIndex``` and ```matchEndIndex``` anytime we get a match, rather than only when we are already in a match.