fix: support zeroValue tag on DeletedAt

[qiankunli]

type Book struct {
	ID        uint
	Name      string
	Pages     uint
	DeletedAt gorm.DeletedAt `gorm:"zeroValue:'1970-01-01 00:00:01'"`
}

it may be necessary for us to set the type of delete_at datetime. and if delete_at is null, we can not add it into the composite index. so we can use 1970-01-01 00:00:01 (or other values you like)as zero value.

if we query books, gorm can generate the sql automatically

select * from books where delete_at = '1970-01-01 00:00:01'

if we delete books, gorm can generate the sql automatically

update books set  delete_at = 'xx' where delete_at = '1970-01-01 00:00:01'