function negotiator(req) {
    var h = req.headers;

    if (!req._negotiator) {
        req._negotiator = new Negotiator({
            headers: {
                accept: h.accept || '*/*',
                'accept-encoding': h['accept-encoding'] || 'identity'
            }
        });
    }

    return req._negotiator;
}
Creates and sets negotiator on request if one doesn't already exist,
then returns it.

@private
@function negotiator
@param    {Object} req - the request object
@returns  {Object}     a negotiator