$dateFromParts should accept an object from another expression

Currently $dateFromParts only accepts explicit objects with required fields. It should be able to accept an expression as long as expression evaluates to an object with the required fields.

This way a date can round trip through $dateToParts and $dateFromParts and end up as the same value (which would have caught SERVER-30524, incidentally).

date == {$dateFromParts:{$dateToParts:{date:"$date"}}}