fix "only" with Relationship fields using marshmallow 2.15.1

[mdodsworth]

This change made in Marshmallow 2.15.1 changes behaviour when the caller is passing through only=():

marshmallow-code/marshmallow@98f2b47#diff-035365c88325ed9779fcb020f7bcb35a

Previously, only=() essentially meant no filtering is applied; after the change, only=() results in all fields being filtered.

Here, we're defaulting the value of only to be (), which then results in all fields being filtered (incl. id). https://github.com/marshmallow-code/marshmallow-jsonapi/blob/dev/marshmallow_jsonapi/fields.py#L104

This causes a bunch of tests to fail with issues similar to this:

self = <CommentSchema(many=False, strict=False)>, args = (), kwargs = {'exclude': (), 'only': ()}

    def __init__(self, *args, **kwargs):
        self.include_data = kwargs.pop('include_data', ())
        super(Schema, self).__init__(*args, **kwargs)
        if self.include_data:
            self.check_relations(self.include_data)

        if not self.opts.type_:
            raise ValueError('Must specify type_ class Meta option')

        if 'id' not in self.fields:
>           raise ValueError('Must have an `id` field')
E           ValueError: Must have an `id` field

[rgant]

The change in marshmallow-code/marshmallow#772 broke my environment as well. Thanks @mdodsworth this fixed the issue in all of my test cases.