Make bulk_update work with postgresql's ArrayField

[arnau126]

Fixes #60 issue.

The problem was that some django db functions like: CONCAT, UPPER, CAST (https://docs.djangoproject.com/en/1.11/ref/models/database-functions/) have a list of arguments, while ArrayField has only one argument which is a list.

In order to distinguish between these two cases, we cast the list of arguments to tuple of arguments.

[coveralls]

Coverage decreased (-1.6%) to 96.667% when pulling 616cea0 on arrayfield into f01e671 on master.

[aykut]

Just published 2.2.0, thank you very much @arnau126