modified `hello.py` [connexion](https://github.com/zalando/connexion/) example to include `Accept` header validation decorator.

hello.py

#!/usr/bin/env python3
import connexion
import flask
from decorator import decorator

@decorator
def requires_header(f: callable, *args, **kwargs):
    accept = flask.request.headers.get('Accept')
    print(accept)
    if not accept or accept != 'application/xml':
        return '400', 400
    return f(*args, **kwargs)


def post_greeting(name: str) -> str:
    return 'Hello {name}'.format(name=name)

@requires_header
def get_user() -> str:
    return 'got user'


if __name__ == '__main__':
    app = connexion.FlaskApp(__name__, port=8080, specification_dir='swagger/')
    app.add_api('helloworld-api.yaml', arguments={'title': 'Hello World Example'} ) #,  validate_responses=True)# strict_validation=True)
    app.run()