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
"""
    This checks the limit passed to view and raises BadRequest if
    limit exceed user configured value
    """
    max_val = conf.getint("api", "maximum_page_limit")  
    fallback = conf.getint("api", "fallback_page_limit")
    if value > max_val:
        return max_val
    if value == 0:
        return fallback
    if value < 0:
        raise BadRequest("Page limit must be a positive integer")
    return value
T = TypeVar("T", bound=Callable)  
def format_parameters(params_formatters: Dict[str, Callable[..., bool]]) -> Callable[[T], T]:
    """
    Decorator factory that create decorator that convert parameters using given formatters.
    Using it allows you to separate parameter formatting from endpoint logic.
    :param params_formatters: Map of key name and formatter function
    """
    def format_parameters_decorator(func: T):
        @wraps(func)
        def wrapped_function(*args, **kwargs):
            for key, formatter in params_formatters.items():
                if key in kwargs:
                    kwargs[key] = formatter(kwargs[key])
            return func(*args, **kwargs)
        return cast(T, wrapped_function)
    return format_parameters_decorator