Fix NpgsqlParameter.Clone() method. It wasn't cloning the npgsqlValue field.

Thanks @Tradioyes for the test and @kukhar for the patch.
Fix #371

(cherry picked from commit d67a54463849e21c01a8bd4eae19cf5299b41696)
From #377
Improved test to check all cloned fields. Thanks @roji for the heads up!

(cherry picked from commit d22213a62682b34f6c428e57b79bae6b36981b97)
From #377