Modify hstore adaptor to only produce strings

In short, the previous implementation would adapt some types into values incompatible with hstore. For example, a dictionary with a boolean value would cause a SQL error. Since hstore only accepts strings (and Null), everything needs to be cast down to a string representation.

Perhaps the question is: Should using non-string types be an error? If so, there should be a better error message. The prior implementation produced a SQL parser error.

This is probably not the best solution, so I would appreciate feedback.

Please see commit message for more detail on the change.