Fix milliseconds field handling when using NpgsqlCopySerializer.AddDateTime() method.

Fix #221
Npgsql wasn't formatting the milliseconds field correctly when sending a DataTime to server.
Thanks @stewienj for the patch.
Merge pull request #319 from franciscojunior/221-fix-adddatetime-milliseconds

Fix milliseconds field handling when using NpgsqlCopySerializer.AddDateTime() method.