[S.R.Serialization] fix nullable serialization to match referencesource codegen.

[S.R.Serialization] fix nullable serialization to match referencesource codegen.

The original code is complicated enough that I misinterpreted the logic...

The reulting code should look like this:

	DateTime? expr_94 = timeSubmitted;
	DateTime dateTime;
	bool arg_B5_0;
	if (XmlObjectSerializerWriteContext.GetHasValue<DateTime> (expr_94)) {
		dateTime = XmlObjectSerializerWriteContext.GetNullableValue<DateTime> (expr_94);
		arg_B5_0 = false;
	}
	else {
		dateTime = XmlObjectSerializerWriteContext.GetDefaultValue<DateTime> ();
		arg_B5_0 = true;
	}

The fixed code should be like this now.