Mongo:should check for createAt field is nil

add such check
```go
if data.CreatedAt.IsZero() {
		data.CreatedAt = time.Now()
	}
```
or add hook in model
```go
func (u *User) Creating() error {
	if u.CreatedAt.IsZero() {
		u.CreatedAt = time.Now()
	}
	return nil
}

func (u *User) Updating() error {
	if u.UpdatedAt.IsZero() {
		u.UpdatedAt = time.Now()
	}
	return nil
}
```