error 500 when using an array in post method.

Hi guys
Back to the issue [why Loopback + MongoDB is not working?](https://github.com/strongloop/loopback-next/issues/3645) another problem I found:

I removed exclude from schema:

```ts
async create(
@requestBody({
content: {
'application/json': {
schema: getModelSchemaRef(Use_r, { exclude: ['id'] }_),
},
},
})
user: Omit,
): Promise {
return this.userRepository.create(user);
}

```

then I add `generated: true` and removed `required: true` from the user model .

```ts
@property({
type: 'number',
id: true,
generated: true
})
id: number;
```

it is looking fine when I used POST method, for one record/document when in add several records it ( array of records )I get:

```json
[{
"email": "nunc.sed@semelit.net",
"password": "571087741"
},
{
"email": "lacus@dui.org",
"password": "346470131"
},
{
"email": "ipsum@aaliquetvel.org",
"password": "243339637"
}]
```

```json
{
"error": {
"statusCode": 500,
"message": "Internal Server Error"
}
}
```

```
Unhandled error in POST /users: 500 TypeError: model.toObject is not a function
at UserRepository.toEntity (D:\apps\test\node_modules\@loopback\repository\src\repositories\legacy-juggler-bridge.ts:471:39)
at UserRepository.create (D:\apps\test\node_modules\@loopback\repository\src\repositories\legacy-juggler-bridge.ts:338:17)
```

But regardless of the error message, it was POSTed all records to MongoDB correctly data with autogenerated id.

## Acceptance criteria

- Improve our REST validation to reject Array values for parameters accepting an object - see #4701
- Modify `DefaultCrudRepository.create` to reject requests to create multiple model instances in a single call

---

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