title: Rails: How is `translate` exposed to ActionController?
body: <p>Can anyone explain how <code>I18n.translate</code> is exposed to the <code>ActionController</code> class and can be used as plain <code>translate</code> or <code>t</code>?  I would like to tweak the <code>translate</code> code but can't understand how it is exposed - for example calling <code>translate.source_location</code> throws an error indicating that there is no method <code>translate</code> in class <code>ActionController</code>.</p>