Previously all entities had internal reference to the root EM - the one created when
initializing the ORM. Now only managed entities (those merged to the EM, e.g. loaded
from the database) have this internal reference.

To use `assign()` method on new (not managed) entities, you need to provide the `em`
parameter:

```typescript
const book = new Book();
wrap(book).assign(data, { em: orm.em });
```

Closes: #267