Fix field_has_choices() when field.choices is an itertools.tee.

Fixes this error:

    File "/home/installuser/Dropbox/projects/ischool/user_manager/django/django-dynamic-fixture/django_dynamic_fixture/django_helper.py", line 138, in field_has_choices
        return bool(len(field.choices) > 0 and field.choices)
    TypeError: object of type 'itertools.tee' has no len()