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
@conf_vars({("api", "maximum_page_limit"): "320"})
    def test_maximum_limit_return_val(self):
        limit = check_limit(300)
        self.assertEqual(limit, 300)
    @conf_vars({("api", "maximum_page_limit"): "320"})
    def test_maximum_limit_returns_configured_if_limit_above_conf(self):
        limit = check_limit(350)
        self.assertEqual(limit, 320)
    @conf_vars({("api", "maximum_page_limit"): "1000"})
    def test_limit_returns_set_max_if_give_limit_is_exceeded(self):
        limit = check_limit(1500)
        self.assertEqual(limit, 1000)
    @conf_vars({("api", "fallback_page_limit"): "100"})
    def test_limit_of_zero_returns_default(self):
        limit = check_limit(0)
        self.assertEqual(limit, 100)
    @conf_vars({("api", "maximum_page_limit"): "1500"})
    def test_negative_limit_raises(self):
        with self.assertRaises(BadRequest):
            check_limit(-1)
class TestFormatParameters(unittest.TestCase):