.hasOwnProperty() does not work on Object.create(null)

[kixxauth]

We need to watch out for cases like this one:

https://github.com/kixxauth/brixx/blob/master/index.js#L68

    return Object.keys(mixin).reduce(function (proto, key) {
      if (!proto.hasOwnProperty(key)) {
        proto[key] = mixin[key];
      }
      return proto;
    }, proto);