Fix to enable compatibility with brutusin/json-forms.
Fix to enable compatibility with brutusin/json-forms.

It looks like json-forms doesn't make any accomodations for multiple
types [1]. Resulting in `Nested(multiple=True)` schemas not rendering
(as of json-forms v1.6.3).

This small change keeps generated schemas valid, and enables some level
of compatibility.

[1] https://github.com/brutusin/json-forms/blob/fcd15f14a7e97003efb12a6b70856c8c582a077b/src/js/brutusin-json-forms.js#L1090

fuhrysteve/marshmallow-jsonschema