Treat DECIMAL like REAL

Hi there,

I had a problem with activerecord-jdbc as it was returning floats as strings.

Example

    ActiveRecord::Base.connection.execute('SELECT 1.99')    

resulted in

    [{"1.99"=>"1.99"}]

Which render arjdbc incompatible with other adapters like mysql2, which would return

    [{"1.99"=>1.99}]

The fix is very light and should not cause any issues.