fix for testing autogenerated keys

it seems that ResultSet#getObject doesn't reliably return an appropriate
datatype when fetching auto-generated keys after an insert statement
eg for Derby, even if the autogenerated column has type INT or BIGINT,
a java.util.BigDecimal is still returned.

The change works around this problem by using one of the ResultSet#get<X>
methods to cast the result to the appropriate datatype, and adds tests for
this problem.
Merge pull request #135 from benilovj/autogenerated-key-type-fix

fix for testing autogenerated keys