Add ability to update JSON fields that are lists in Model.bulk_update
I've patched this in a slightly different way. Am hoping to extend the code elsewhere to use the new `to_value()` interface (as opposed to manually creating Value instances w/the appropriate converter). Thx for the patch, at any rate.
I noticed that using `Model.bulk_update` won't work with `JSONField`s that are top-level lists because by default `Value` will unpack the list into multiple values. Top-level lists are valid JSON objects.

Example (before):
```python
class Item(Model):
    json = JSONField()

item = Item.create(json=['a','b'])
item.json = ['c','d']

Item.bulk_update([item], fields=[Item.json])
```
will raise `OperationalError` because of a malformed update query.

Example (after):
```python
class Item(Model):
    json = JSONField()

item = Item.create(json=['a','b'])
item.json = ['c','d']

Item.bulk_update([item], fields=[Item.json], unpack_values=False)
```
will update the row fine.

This applies to both SQLite and PostgreSQL `JSONField`s.