Return super in ActionController::Parameters.const_missing

The current implementation of ActionController::Parameters.const_missing
returns `ActionController::Parameters.always_permitted_parameters` even
if its `super` returns a constant without raising error. This prevents its
subclass in a autoloading module/class from taking advantage of
autoloading constants.

    class SomeParameters < ActionController::Parameters
      def do_something
        DefinedSomewhere.do_something
      end
    end

In the code above, `DefinedSomewhere` is to be autoloaded with
`Module.const_missing` but `ActionController::Parameters.const_missing`
returns `always_permitted_parameters` instead of the autoloaded
constant.

This pull request fixes the issue respecting `const_missing`'s `super`.

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -117,7 +117,7 @@ class Parameters < ActiveSupport::HashWithIndifferentAccess
     self.always_permitted_parameters = %w( controller action )
 
     def self.const_missing(const_name)
-      super unless const_name == :NEVER_UNPERMITTED_PARAMS
+      return super unless const_name == :NEVER_UNPERMITTED_PARAMS
       ActiveSupport::Deprecation.warn(<<-MSG.squish)
         `ActionController::Parameters::NEVER_UNPERMITTED_PARAMS` has been deprecated.
         Use `ActionController::Parameters.always_permitted_parameters` instead.

actionpack/test/controller/parameters/always_permitted_parameters_test.rb

 require 'abstract_unit'
 require 'action_controller/metal/strong_parameters'
+require 'minitest/mock'
 
 class AlwaysPermittedParametersTest < ActiveSupport::TestCase
   def setup
@@ -14,7 +15,13 @@ def teardown
 
   test "shows deprecations warning on NEVER_UNPERMITTED_PARAMS" do
     assert_deprecated do
-       ActionController::Parameters::NEVER_UNPERMITTED_PARAMS
+      ActionController::Parameters::NEVER_UNPERMITTED_PARAMS
+    end
+  end
+
+  test "returns super on missing constant other than NEVER_UNPERMITTED_PARAMS" do
+    ActionController::Parameters.superclass.stub :const_missing, "super" do
+      assert_equal "super", ActionController::Parameters::NON_EXISTING_CONSTANT
     end
   end