Fixes defect in incrementing the index

If the dataset already has index and the save function demands creating
a new set of indexes [ i.e. making a new index key], then this change
helps in doing so.
To support the claim, I have created a new test case simulating the
scenario.

* Whether we have to drop the previous index / not is something we will
observe & accomodate
Forgot to commit this version number
Merge branch 'master' of github.com:shyamperi/scraperwiki-ruby into shyamperi-master
New version