def METHOD_NAME(get_proxies, post, get_canonical_facts):
    """
    A POST requests to the check-in endpoint is issued with correct headers and
    body containing Canonical Facts.
    """
    config = Mock(base_url="www.example.com")
    connection = InsightsConnection(config)
    connection.checkin()
    expected_url = connection.inventory_url + "/hosts/checkin"
    expected_headers = {"Content-Type": "application/json"}
    expected_data = get_canonical_facts.return_value
    post.assert_called_once_with(
        expected_url, headers=expected_headers, data=dumps(expected_data)
    )