Add Date and DateTime CFs to bulk update

Date conversion is needed only for delete because AddValueForObject
in RT::CustomField calls CanonicalizeValue methods to handle
conversion on add.

For Date and DateTime, pass undef for Default to assure that we
don't incorrectly imply that there will be a default value set
if the box is left empty. This inconsistency has been resolved in
4.4 via 4.4/date-input-keep-value

@values is checked for length, otherwise empty string values
are submitted for empty fields resulting in update messages
noting "Not set changed to Not set".
Merge branch '4.0/add-date-datetime-to-bulk-update' into 4.0-trunk