Revert "Convert StrongParameters cache to a hash. This fixes an unbounded"

We cannot cache keys because arrays are mutable. We rather want to cache
the arrays. This behaviour is tailor-made for the usage pattern strongs
params is designed for.

In a forthcoming commit I am going to add a test that covers why we need
to cache by value.

Every strong params instance has a live span of a request, the cache goes
away with the object. Since strong params have such a concrete intention,
it would be interesting to see if there are actually any real-world use
cases that are an actual leak, one that practically may matter.

I am not convinced that the theoretical leak has any practical consequences,
but if it can be shown there are, then I believe we should either get rid of
the cache (which is an optimization), or else wipe it in the mutating API.

This reverts commit e63be276.

actionpack/lib/action_controller/metal/strong_parameters.rb

@@ -130,7 +130,7 @@ def initialize(attributes = nil)
     # looping in the common use case permit + mass-assignment. Defined in a
     # method to instantiate it only if needed.
     def converted_arrays
-      @converted_arrays ||= {}
+      @converted_arrays ||= Set.new
     end
 
     # Returns +true+ if the parameter is permitted, +false+ otherwise.
@@ -333,15 +333,15 @@ def permitted=(new_permitted)
 
     private
       def convert_hashes_to_parameters(key, value, assign_if_converted=true)
-        converted = convert_value_to_parameters(key, value)
+        converted = convert_value_to_parameters(value)
         self[key] = converted if assign_if_converted && !converted.equal?(value)
         converted
       end
 
-      def convert_value_to_parameters(key, value)
-        if value.is_a?(Array) && !converted_arrays.member?(key)
-          converted = value.map { |v| convert_value_to_parameters(nil, v) }
-          converted_arrays[key] = converted if key
+      def convert_value_to_parameters(value)
+        if value.is_a?(Array) && !converted_arrays.member?(value)
+          converted = value.map { |_| convert_value_to_parameters(_) }
+          converted_arrays << converted
           converted
         elsif value.is_a?(Parameters) || !value.is_a?(Hash)
           value

actionpack/test/controller/parameters/parameters_permit_test.rb

@@ -169,7 +169,7 @@ def assert_filtered_out(params, key)
 
   test 'arrays are converted at most once' do
     params = ActionController::Parameters.new(foo: [{}])
-    assert_same params[:foo], params[:foo]
+    assert params[:foo].equal?(params[:foo])
   end
 
   test "fetch doesnt raise ParameterMissing exception if there is a default" do