Model.update not detecting datetime change in FastAPI

# Bug

I'm trying to automatically update the _modified_at_ field in my **Server** model from a FastAPI enpoint, but the **Model.update** method doesn't update the field.

### Current Behavior

For this i have the base model to **create** a Server (simplified version):

```
class Server(Model):
id: UUID = Field(primary_field=True)
name: str
created_at: datetime = Field(default_factory=datetime.now)
modified_at: datetime = Field(default_factory=datetime.now)
```
and a model used to **patch** the Server that automatically updates the _modified_at_ field (simplified version):

```
class ServerUpdate(BaseModel):
name: Optional[str]
modified_at: Optional[datetime]

class Config:
validate_assignment = True

@root_validator
def number_validator(cls, values):
values["modified_at"] = datetime.now()
return values
```

In my FastAPI endpoint i have something like this:

```
@router.put("/{id}", response_model=Server)
async def update_server_by_id(id: UUID, patch: ServerUpdate):
server = await engine.find_one(Server, Server.id == id)

if server is None:
raise HTTPException(404)

server.update(patch)

try:
await engine.save(server)
return server
except Exception:
raise HTTPException(500)
```
The _modified_at_ field is **never updated**.

If I `print(patch.dict())` it gives me:
`{'name': None, 'modified_at': datetime.datetime(2022, 9, 11, 11, 4, 6, 319829)}`

If I `print(server.dict())` before the update it gives me:
`{'id': UUID('c4dfa7ac-d25d-45cb-8bc3-f59e5938a58c'), 'name': 'Server 1', 'created_at': datetime.datetime(2022, 9, 11, 9, 46, 45, 234000), 'modified_at': datetime.datetime(2022, 9, 11, 9, 46, 45, 234000)}`

so the **patch** instance has the current datetime.

The only way I found to make it work is to set **`patch.modified_at = patch.modified_at`** before `server.update(patch)`

### Expected behavior

The Model.update method should update the datetime field.

### Environment

- ODMantic version: 0.8.0
- MongoDB version: 5.0.12
- Pydantic infos (output of `python -c "import pydantic.utils; print(pydantic.utils.version_info())`):

```
pydantic version: 1.10.1
pydantic compiled: True
install path: D:\Projects\Python\vendor-system-fastapi\.venv\Lib\site-packages\pydantic
python version: 3.10.6 (tags/v3.10.6:9c7b4bd, Aug 1 2022, 21:53:49) [MSC v.1932 64 bit (AMD64)]
platform: Windows-10-10.0.19043-SP0
optional deps. installed: ['dotenv', 'typing-extensions']
```

- Version of additional modules (if relevant):
- fastapi: 0.82.0

**Additional context**

_Add any other context about the problem here._

Start by reproducing the FastAPI update_server_by_id endpoint with the Server and ServerUpdate examples, then trace Model.update in ODMantic 0.8.0. Verify that modified_at is persisted by engine.save(server), and add a regression test covering the datetime change.

Written by the indexing model from the issue text.