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

Thanks @Tradioyes for the test and @kukhar for the patch.
Fix #371
Improved test to check all cloned fields. Thanks @roji for the heads up!
Merge pull request #377 from franciscojunior/371-fix-npgsqlparameter-clone-copy-npgsqlvalue

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