Revert "Return true from NpgsqlDataReader.HasRows even after all rows have been read."

This reverts commit c7b5c36bbe79bcf5a30d60153e6b6ba99b66b20b.
Merge pull request #180 from franciscojunior/revert-c7b5c36bbe

Revert "Return true from NpgsqlDataReader.HasRows even after all rows ha...