Triggering a dagrun with naive datetime raises an HTML page instead of JSON

**Apache Airflow version**:2.0


**What happened**:

An HTML page was returned instead of json when the REST API was triggered with a naive datetime

**What you expected to happen**:

I expected it to raise error status 400 indicating that it's a bad request

**How to reproduce it**:
1. Start the web server and scheduler inside breeze.
2. Make a post request to this endpoint: http://localhost:28080/api/v1/dags/example_bash_operator/dagRuns
 to trigger dagruns using this request body: {"execution_date": "2020-11-09T16:25:56.939143"}. or any naive datetime
3. See that an HTML is raised instead of Json

tests/api_connexion/endpoints/test_dag_run_endpoint.py
{
                    "detail": "Property is read-only - 'start_date'",
                    "status": 400,
                    "title": "Bad Request",
                    "type": EXCEPTIONS_LINK_MAP[400],
                },
            ),
            (
                "state in request json",
                "api/v1/dags/TEST_DAG_ID/dagRuns",
                {"state": "failed", "execution_date": "2020-06-12T18:00:00+00:00"},
                {
                    "detail": "Property is read-only - 'state'",
                    "status": 400,
                    "title": "Bad Request",
                    "type": EXCEPTIONS_LINK_MAP[400],
                },
            ),
        ]
    )
    @provide_session
    def test_response_400(self, name, url, request_json, expected_response, session):
        del name
        dag_instance = DagModel(dag_id="TEST_DAG_ID")
        session.add(dag_instance)
        session.commit()
        response = self.client.post(url, json=request_json, environ_overrides={'REMOTE_USER': "test"})
        self.assertEqual(response.status_code, 400, response.data)
        self.assertEqual(expected_response, response.json)
    def test_response_409(self):