def _activity_form_to_outgoing_string(activity_form):
    """
    Formats an activity ready to be sent out as
    an HTTP response.
    """
    from kepi.bowler_pub import ATSIGN_CONTEXT
    from kepi.bowler_pub.utils import as_json
    format_for_delivery = activity_form.copy()
    for blind_field in ['bto', 'bcc']:
        if blind_field in format_for_delivery:
            del format_for_delivery[blind_field]
    if '@context' not in format_for_delivery:
        format_for_delivery['@context'] = ATSIGN_CONTEXT
    message = as_json(format_for_delivery)
    return message
