union(distinct_id, data, modifiers, callback) {
        const identifiers = {$distinct_id: distinct_id};
        this._union({identifiers, data, modifiers, callback});
    }
people.union(distinct_id, data, modifiers, callback)
        ---
        merge value(s) into a list-valued people analytics property.

        usage:

            mixpanel.people.union('bob', {'browsers': 'firefox'});

            mixpanel.people.union('bob', {'browsers': ['chrome'], os: ['linux']});