Fix syntax error in postgresql_user

##### Issue Type:
Bugfix Pull Request

##### Ansible Version:
devel

##### Environment:
Arch, Ubuntu 14.04

##### Summary:
postgresql_user module fails with error `AttributeError: 'frozenset' object has no attribute 'is_subset'`

##### Steps To Reproduce:
`ansible somehost -m postgresql_user -a "name=someuser state=present"`

##### Expected Results:
`someuser` is created in the database

##### Actual Results:
fails with error `AttributeError: 'frozenset' object has no attribute 'is_subset'`