ActionMailer undefined method `perform_caching'

It is impossible to include a partial with cache into email to be sent using ActionMailer.

```
ActionView::Template::Error (undefined method `perform_caching' for #<ContactMailer:0x007f8dc14f5578>):
1: <% cache [object, admin?] do %>
...
```

So I have added a helper to the mailer:

```
helper_method :cache
def cache *args
  yield
end
```

But this seems a bit half-assed.
