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

airflow/api_connexion/parameters.py
#
#
from functools import wraps
from typing import Callable, Dict, TypeVar, cast
from pendulum.parsing import ParserError
from airflow.api_connexion.exceptions import BadRequest
from airflow.configuration import conf
from airflow.utils import timezone
def format_datetime(value: str):
    """
    Datetime format parser for args since connexion doesn't parse datetimes
    https://github.com/zalando/connexion/issues/476
    This should only be used within connection views because it raises 400
    """
    value = value.strip()
    if value[-1] != 'Z':
        value = value.replace(" ", '+')
    try:
        return timezone.parse(value)
    except (ParserError, TypeError) as err:
        raise BadRequest("Incorrect datetime argument", detail=str(err))
def check_limit(value: int):