Fix *ambigious match found* exception for PostgreSQL.

Got this exception when using small integer value inside `where` clause in query expression. I think the issue is described under *Remarks* section at Npgsql reference for [`NpgsqlParameter(string, object)` constructor overload](http://npgsql.projects.pgfoundry.org/docs/api/Npgsql.NpgsqlParameterConstructor2.html).