Clone `user` because adapters may touch it later

This fixes a bug that cron job sometime sends message to unexpected room.

Adapters may change user.room and reply_to later, with the last room
where an user speaked.

e.g. https://github.com/hipchat/hubot-hipchat/blob/3cd23598c6a33c827dbe286fe673287e2d642fb8/src/hipchat.coffee#L131-L133
Merge pull request #6 from sorah/clone_user

Clone `user` because adapters may touch it later