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
#
#
import unittest
from unittest import mock
from pendulum import DateTime
from pendulum.tz.timezone import Timezone
from airflow.api_connexion.exceptions import BadRequest
from airflow.api_connexion.parameters import check_limit, format_datetime, format_parameters
from airflow.utils import timezone
from tests.test_utils.config import conf_vars
class TestDateTimeParser(unittest.TestCase):
    def setUp(self) -> None:
        self.default_time = '2020-06-13T22:44:00+00:00'
        self.default_time_2 = '2020-06-13T22:44:00Z'
    def test_works_with_datestring_ending_00_00(self):
        datetime = format_datetime(self.default_time)
        datetime2 = timezone.parse(self.default_time)
        assert datetime == datetime2
        assert datetime.isoformat() == self.default_time
    def test_works_with_datestring_ending_with_zed(self):
        datetime = format_datetime(self.default_time_2)
        datetime2 = timezone.parse(self.default_time_2)
        assert datetime == datetime2
        assert datetime.isoformat() == self.default_time  
    def test_raises_400_for_invalid_arg(self):
        invalid_datetime = '2020-06-13T22:44:00P'
        with self.assertRaises(BadRequest):
            format_datetime(invalid_datetime)
class TestMaximumPagelimit(unittest.TestCase):