set(distinct_id, prop, to, modifiers, callback) {
        const identifiers = {$distinct_id: distinct_id};
        this._set(prop, to, modifiers, callback, {identifiers});
    }
people.set(distinct_id, prop, to, modifiers, callback)
        ---
        set properties on an user record in engage

        usage:

            mixpanel.people.set('bob', 'gender', 'm');

            mixpanel.people.set('joe', {
                'company': 'acme',
                'plan': 'premium'
            });