API call /flow_manager/v2/flows/{dpid} on DELETE is returning misleading response status

[ArturoQuintana]

@hdiogenes @italovalcy @ajoaoff @rmotitsuki

• If the payload items field is empty, it returns 200 when it should be 400.

reflected on the end-to-end test:
tests/test_e2e_20_flow_manager.py::TestE2EFlowManager::test_022_delete_flow_should_fail

Code:

def test_022_delete_flow_should_fail(self):
        """Test if the flow deletion process specifying an unknown path behaves as expected (400 Error)."""

        payload = {}

        # delete the flow
        api_url = KYTOS_API + '/flow_manager/v2/flows/00:00:00:00:00:00:00:01'
        response = requests.delete(api_url, data=json.dumps(payload),
                                   headers={'Content-type': 'application/json'})
        assert response.status_code == 400

The same happens with these cases:

payload = {
            "flows": [
            ]
        }

and

payload = {
            "flows": [
                {

                }
            ]
        }

• A not serializable JSON broke the application, and it should return a 400 error.

Case:

payload = {
            "flows": [
                {
                    "priority"
                }
            ]
        }

The issue mentioned above should be verified for any not serializable JSON