Issue-172: Fix for connection.indexes not returning proper unique setting in Postgres

This PR fixes #172. The connection.indexes will now return the correct information about the uniqueness of an index.
I did the fix this way because I was not sure what the use case was that prompted the need to check if is_unique == 't' for it to mark unique as true. Looking at the current ActiveRecord code for indexes it simply takes whatever value is returned by the query in the map. So if there is no reason to check to see if is_unique == 't', then this code can be improved by dropping the setting of unique all together and changing unique to is_unique on line 198.