title: What is code for devise authenticate_user! after generated for :user
body: <p>I realize that <code>authenticate_user!</code> is <a href="https://stackoverflow.com/questions/9272272/where-is-devise-implementation-of-authenticate-user-method">not explicitly defined</a> in the gem files, but I am wondering for the typical app (authentication on model named <code>User</code>), what would the method look like. I need to know so that I can modify it slightly.  </p>