EmitDefaultValue=false doesn't work on value types

[mfenniak]

It doesn't seem to work to have a Guid "ID" field, because EmitDefaultValue doesn't work correctly on Guid types in the datum converter.

        [DataMember(Name = "id", EmitDefaultValue = false)]
        public Guid Id;

causes

[ERROR] FATAL UNHANDLED EXCEPTION: System.AggregateException:  ---> System.InvalidProgramException: Invalid IL code in DataContractDatumConverterFactory.RethinkDb.Examples.ConsoleApp.Person:ConvertObject (RethinkDb.Examples.ConsoleApp.Person): IL_0031: ceq