don't mutate hash with fetch

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -284,7 +284,14 @@ def [](key)
     #   params.fetch(:none, 'Francesco')    # => "Francesco"
     #   params.fetch(:none) { 'Francesco' } # => "Francesco"
     def fetch(key, *args)
-      convert_hashes_to_parameters(key, super)
+      value = super
+      # Don't rely on +convert_hashes_to_parameters+
+      # so as to not mutate via a +fetch+
+      if value.is_a?(Hash)
+        value = self.class.new(value)
+        value.permit! if permitted?
+      end
+      value
     rescue KeyError
       raise ActionController::ParameterMissing.new(key)
     end

actionpack/test/controller/parameters/parameters_permit_test.rb

@@ -147,6 +147,12 @@ def assert_filtered_out(params, key)
     assert_equal :foo, e.param
   end
 
+  test "fetch with a default value of a hash does not mutate the object" do
+    params = ActionController::Parameters.new({})
+    params.fetch :foo, {}
+    assert_equal nil, params[:foo]
+  end
+
   test "fetch doesnt raise ParameterMissing exception if there is a default" do
     assert_equal "monkey", @params.fetch(:foo, "monkey")
     assert_equal "monkey", @params.fetch(:foo) { "monkey" }