Added milliseconds if field_type is Time

Using :created_at as 'field_name'.

`Item.desc(:created_at).limit(IMOS_PER_PAGE).scroll(current_cursor) do |item, cursor|`

My items are created very fast and I have several of them created at the same second. The cursor just contemplated seconds and whenever a new cursor was passed, it went straight at the next second. Now it takes milliseconds in :created_at. It is more accurate. 

You should refactor this out of this class, possibly as an extension to `DateTime` and/or `Time`, and it should have tests of its own.