Post_collection is not raising BadRequestError when payload relationships key's value are not a dictionary

[kaitj91]

It is very important that the library is able to return some sort of response that let's the user know they submitted an invalid request by raising a BadRequestError and returning a 400 and some sort of description explaining why.

Through testing post_collection, I found that we are not raising a BadRequestError when we send a payload where the relationships key's value are not of type dictionary.
For example:

    def test_add_resource_when_payload_relationships_keys_value_are_not_type_dict(self):
        """Create a resource when payload relationships key's are not of type dictionary returns 400.

        Each relationships key's value must be of type dictionary.
        A BadRequestError is raised.
        """
        user = models.User(
            first='Sally', last='Smith',
            password='password', username='SallySmith1')
        self.session.add(user)
        self.session.commit()

        payload = {
            'data': {
                'attributes': {
                    'first': 'Sally',
                    'last': 'Smith',
                    'username': 'SallySmith1',
                    'password': 'password',
                },
                'type': 'users',
                'relationships': {
                    'posts': [{
                        'data': [{
                            'type': 'posts',
                            'id': 1
                        }]
                    }]
                }
            }
        }

        with self.assertRaises(errors.BadRequestError) as error:
            models.serializer.post_collection(
                self.session, payload, 'users')

        self.assertEqual(
            error.exception.detail, 'relationship posts must be an object')
        self.assertEqual(error.exception.status_code, 400)

This test fails because there is no check for this in place in post_collection. Thus when I try to run this test a AttributeError occurs. Specifically the AttributeError is occuring because we are failing to check

if not isinstance(data_rel, dict):
    raise BadRequestError('relationship {0} must be an object'.format(api_key))

before executing this piece of code in post_collection.

                if 'data' not in data_rel.keys():
                    raise BadRequestError(
                        'Missing data key in relationship {}'.format(key))

Because we do not have this check, a AttributeError occurs because just prior to this code we have:

                data_rel = data['data']['relationships'][api_key]

Thus when trying to do data_rel.keys() when data_rel is actually a list, results in an AttributeError since a 'list' object does not have the attribute 'keys'.
This is an easy fix that would provide an informative error message to users who send badly formatted payloads.