Clean up uniques when deleting

A bug in the test where the key "unique" was used 
instead of "uniques" meant the test 
"removes the previous index when deleting" was
passing when it was in fact failing. Model.delete 
now mirrors Model.save in how it cleans up indexes
and uniques.
Merge pull request #86 from damonmorgan/bug/delete_unique

Clean up uniques when deleting