(Date/Time support) Allow update operators on Date/Time valued fields

[tatu-at-datastax]

As part of Date/Time (#362) support we need to ensure that existing update operators applicable to this type (generally same as other non-numeric types) can be used with Date/Time fields.
This may require changes to update operators, especially validation (since pseudo-field name $date needs to be allowed and not taken as update operator); but even without other changes need to add testing.