Support DateTime values

[cap1024]

Please add support DateTime values. Following example produces NullReferenceException:

            Action<EntityTypeBuilder<SourceEntity>> builder = x =>
                x.AfterUpdate(trigger => trigger
                    .Action(action => action
                        .Update<DestinationEntity>(
                            (tableRefs, destinationEntities) 
                                => destinationEntities.StringField == tableRefs.New.StringField + tableRefs.Old.StringField,
                            (tableRefs, destinationEntities) 
                                => new DestinationEntity
                                {
                                    DateTimeValue = DateTime.UtcNow
                                })));

Useful case:

            MofiedOn = DateTime.UtcNow

Just to save time to search for a place with an error:

    public sealed class MemberExpressionVisitor : BaseExpressionVisitor<MemberExpression>
    {
        private string GetTableSql(MemberExpression memberExpression, ArgumentType argumentType)
        {
            if (memberExpression.Member.TryGetNewTableRef(out _))
            {
                return _generator.NewEntityPrefix;
            }
            if (memberExpression.Member.TryGetOldTableRef(out _))
            {
                return _generator.OldEntityPrefix;
            }
            return memberExpression.Expression != null
                ? GetColumnSql(memberExpression.Expression.Type, memberExpression.Member, argumentType)
                : GetMethodSql(memberExpression);
        }
        private string GetMethodSql(MemberExpression memberExpression)
        {
            var method = memberExpression.ToString();
            switch (method)
            {
                case "String.Empty": return _generator.GetSql(string.Empty);
                case "DateTime.UtcNow": return "GetUtcDate()"; // for SQL Server
            }
            throw new InvalidOperationException($"Unknown method: {method}");
        }
    }

[win7user10]

Hello. I added support for DateTime.UtcNow in the latest version.

[cap1024]

Hello! Thank a lot! I've added PR with minor changes. Pls, review it.