Interfaces are ignored when define Mutation.

Inizia in graphene/types/mutation.py, in init_subclass_with_meta, e riproduci l'esempio CreatePlan segnalato. La correzione è completata quando la definizione di una mutation con interfacce conserva i campi delle interfacce nel suo output invece di scartarli quando non viene fornito alcun output esplicito.

Hello.
First of all, thanks for all graphene developers and contributors.

• What is the current behavior?
  Interface is ignored when define mutation. This is my code.

class CreatePlan(Mutation):
    class Meta:
        interfaces = (PlanInterface, )

    class Arguments:
        name = String(required=True)
        comment = String(default_value='')
        goal = String(required=True)
        start_date = DateTime(required=True)
        end_date = DateTime(required=True)

    def mutate(parent, info, **kwargs):
        ~~~
# this raises AssertionError:
# CreatePlan fields must be a mapping (dict / OrderedDict)
# with field names as keys or a function which returns such a mapping.

• What is the expected behavior?
  I think Mutation should includes all interfaces and fields together.

• Please tell us about your environment:
  Here is my environment.

  • Version: 2.1.9
  • Platform: Mac OS

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow)
  I think removing just a line can resolve this problem.
  branch v2, graphene/types/mutation.py line 69

    @classmethod
    def __init_subclass_with_meta__(
        cls,
        interfaces=(),
        resolver=None,
        output=None,
        arguments=None,
        _meta=None,
        **options
    ):
        if not _meta:
            _meta = MutationOptions(cls)

        output = output or getattr(cls, "Output", None)
        fields = {}

        for interface in interfaces:
            assert issubclass(interface, Interface), (
                'All interfaces of {} must be a subclass of Interface. Received "{}".'
            ).format(cls.__name__, interface)
            fields.update(interface._meta.fields)

        if not output:
            # If output is defined, we don't need to get the fields
            fields = OrderedDict().  # This ignores interfaces. Remove this.
            for base in reversed(cls.__mro__):
                fields.update(yank_fields_from_attrs(base.__dict__, _as=Field))
            output = cls

Also, I found same problem on master branch.

    @classmethod
    def __init_subclass_with_meta__(
        cls,
        interfaces=(),
        resolver=None,
        output=None,
        arguments=None,
        _meta=None,
        **options,
    ):
        if not _meta:
            _meta = MutationOptions(cls)
        output = output or getattr(cls, "Output", None)
        fields = {}

        for interface in interfaces:
            assert issubclass(
                interface, Interface
            ), f'All interfaces of {cls.__name__} must be a subclass of Interface. Received "{interface}".'
            fields.update(interface._meta.fields)
        if not output:
            # If output is defined, we don't need to get the fields
            fields = {}. # This ignores interfaces. Remove this.
            for base in reversed(cls.__mro__):
                fields.update(yank_fields_from_attrs(base.__dict__, _as=Field))
            output = cls