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/test_parameters.py
def test_should_works_with_datetime_formatter(self):
        decorator = format_parameters({"param_a": format_datetime})
        endpoint = mock.MagicMock()
        decorated_endpoint = decorator(endpoint)
        decorated_endpoint(param_a='2020-01-01T0:0:00+00:00')
        endpoint.assert_called_once_with(param_a=DateTime(2020, 1, 1, 0, tzinfo=Timezone('UTC')))
    def test_should_propagate_exceptions(self):
        decorator = format_parameters({"param_a": format_datetime})
        endpoint = mock.MagicMock()
        decorated_endpoint = decorator(endpoint)
        with self.assertRaises(BadRequest):
            decorated_endpoint(param_a='XXXXX')
    @conf_vars({("api", "maximum_page_limit"): "100"})
    def test_should_work_with_limit(self):
        decorator = format_parameters({"limit": check_limit})
        endpoint = mock.MagicMock()
        decorated_endpoint = decorator(endpoint)
        decorated_endpoint(limit=89)
        endpoint.assert_called_once_with(limit=89)