Ensure that not all helpers are loaded for subclasses of ActionController::Base.

Hi,

I came across this issue while upgrading my app to Rails 3.
ActionController::Base is defining an inherited block that tells all
ancestors to use `helper :all`. This is not the right default
behaviour for controllers. In most views you'll simply want to have
the designated helper available and the application helper.

By calling `helper :all`, all modules from app/helpers are sorted
alphabetically and then included in the view. This has the side effect
that the designated helper for this controller (ie. ProductsHelper for
ProductsController) may not be the first helper in the scope of your
view. This becomes a problem because the app I'm upgrading has
conventions whereby a lot of helpers define a #title helper, for
instance, that behaves differently in different views. By calling
`helper :all` in my controller there is a good chance that the helper
call will be sent to the wrong module, giving me the wrong result.

Here is how to reproduce the issue on the gem version of 3.0.0.rc:
http://gist.github.com/517669

I've opened a ticket on lighthouse (and included a patch). Technically
the patch doesn't include a new test, but I added a method to one of
the existing helper modules defined in the fixtures. Without my change
one of the existing tests will fail because it will call the wrong
helper method.

Ticket is https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/5348-visibility-of-helpers-seems-all-wrong

Jesse