asRecord support for single-case discriminated unions

[spiffytech]

Please add support for asRecord deserializing fields into single-case discriminated unions. For example, I have a type Int6 = Int6 of int, which I just use for ints 0 <= i <= 5. When I try to deserialize my int32 database field through asRecord, I get:

System.ArgumentException: Object type System.Int32 cannot be converted to target type: sleep_log.DomainTypes+Int6

I make extensive use of single-case discriminated unions, so having asRecord see that e.g., my int32 database field corresponds to a T<int> record field would be super cool.

[mausch]

Sounds good, wanna give it a try?