Marshmallow validate.Equal does not generate an enum in swagger

[Bangertm]

I would have expected that the marshmallow Equal validator to generate an enum with a single entry in swagger and be the equivalent of OneOf with only a single entry in the list.

For example:

from pprint import pprint
from apispec import APISpec

from marshmallow import Schema, fields
from marshmallow.validate import OneOf, Equal

class MySchema(Schema):
    property_1 = fields.Str(validate=OneOf(['only_choice']))

    property_2 = fields.Str(validate=Equal('only_choice'))

spec = APISpec(title='Demo',
               version='1',
               info=dict(description='demo'),
               plugins=['apispec.ext.marshmallow'])

spec.definition('MySchema', schema=MySchema)

pprint(spec.to_dict())

produces:

{'definitions': {'MySchema': {u'properties': {'property_1': {u'enum': ['only_choice'], u'type': u'string'}, 'property_2': {u'type': u'string'}},
                              u'type': u'object'}},
 'info': {'description': 'demo', 'title': 'Demo', 'version': '1'},
 'parameters': {},
 'paths': OrderedDict(),
 'swagger': '2.0',
 'tags': []}

[sloria]

Good point. I would certainly review and merge a PR for this.