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
#
#
import unittest
from datetime import timedelta
from parameterized import parameterized
from airflow.api_connexion.exceptions import EXCEPTIONS_LINK_MAP
from airflow.models import DagModel, DagRun
from airflow.security import permissions
from airflow.utils import timezone
from airflow.utils.session import create_session, provide_session
from airflow.utils.types import DagRunType
from airflow.www import app
from tests.test_utils.api_connexion_utils import assert_401, create_user, delete_user
from tests.test_utils.config import conf_vars
from tests.test_utils.db import clear_db_dags, clear_db_runs
class TestDagRunEndpoint(unittest.TestCase):