Using sets for choices in StringField(choices=...) raises an error when validating

[bagerard]

The choices parameter does not manage sets properly due to the use indexing (choices[0]).

def test_choices_allow_using_sets_as_choices(self):
    """Ensure that sets can be used when setting choices
        """
    class Shirt(Document):
        size = StringField(choices={'M', 'L'})

  Shirt(size='M').validate()

File "/home/bgerard/dev/mongoengine/tests/fields/fields.py", line 3186
...
self = <mongoengine.fields.StringField object at 0x7ff8be807d50>, value = 'M'

def _validate_choices(self, value):
    Document = _import_class('Document')
    EmbeddedDocument = _import_class('EmbeddedDocument')

    choice_list = self.choices

  if isinstance(choice_list[0], (list, tuple)):

E TypeError: 'set' object does not support indexing

[wojcikstefan]

@bagerard could you please review #1483?

[wojcikstefan]

Ah, apologies, just saw that you submitted #1482. Will close mine, thanks!