Attempted fix for adding an index to a datastore with indexes

I have a datastore with indexes, and when I update the datastore by adding indexes, I get an error on the line store.createIndex... that an index with the given name already exists.

I had a lot of trouble getting the test suite to run as I mentioned in issue #61, but I believe this test would fail and that this fix solves it. Please test it yourself before merging, the travis tests don't seem to run on forked repos or pull requests.