bulk_update for datetime fail
Im always surprised people can take the time to open the issue and then completely fail to explain their actual issue. oh my god stupid thing to do,When I try to use "bulk_update" function update mysql_db datetime field, it doesn't take effect It works fine for me:

```python
class Reg(Model):
    key = CharField()
    timestamp = DateTimeField()

# Create 3 objects w/timestamp of Mar 1, 2, and 3rd.
regs = [Reg.create(key='k%s' % i, timestamp=dt(2022, 3, i))
        for i in (1, 2, 3)]

# Change timestamps to be Apr 1, 2, and 3.
for reg in regs:
    reg.timestamp = reg.timestamp.replace(month=4)

# Perform update.
Reg.bulk_update(regs, fields=[Reg.key, Reg.timestamp])

# Verify timestamps updated correctly.
for reg in Reg.select().order_by(Reg.key):
    print(reg.key, reg.timestamp)
```

Output

```
k1 2022-04-01 00:00:00
k2 2022-04-02 00:00:00
k3 2022-04-03 00:00:00
```

![aaa](https://user-images.githubusercontent.com/30220658/155965562-7511c03e-5de1-4df0-91b1-805303546976.png)


