Define transform_keys! in HashWithIndifferentAccess

Make sure that when transforming the keys of a HashWithIndifferentAccess
we can still access with indifferent access in Ruby 2.5.

Closes #32007.

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -311,6 +311,14 @@ def transform_keys(*args, &block)
       dup.tap { |hash| hash.transform_keys!(*args, &block) }
     end
 
+    def transform_keys!
+      return enum_for(:transform_keys!) { size } unless block_given?
+      keys.each do |key|
+        self[yield(key)] = delete(key)
+      end
+      self
+    end
+
     def slice(*keys)
       keys.map! { |key| convert_key(key) }
       self.class.new(super)

activesupport/test/hash_with_indifferent_access_test.rb

@@ -404,6 +404,12 @@ def test_indifferent_transform_keys
 
     assert_equal({ "aa" => 1, "bb" => 2 }, hash)
     assert_instance_of ActiveSupport::HashWithIndifferentAccess, hash
+
+    hash = ActiveSupport::HashWithIndifferentAccess.new(@strings).transform_keys { |k| k.to_sym }
+
+    assert_equal(1, hash[:a])
+    assert_equal(1, hash["a"])
+    assert_instance_of ActiveSupport::HashWithIndifferentAccess, hash
   end
 
   def test_indifferent_transform_keys_bang
@@ -412,6 +418,13 @@ def test_indifferent_transform_keys_bang
 
     assert_equal({ "aa" => 1, "bb" => 2 }, indifferent_strings)
     assert_instance_of ActiveSupport::HashWithIndifferentAccess, indifferent_strings
+
+    indifferent_strings = ActiveSupport::HashWithIndifferentAccess.new(@strings)
+    indifferent_strings.transform_keys! { |k| k.to_sym }
+
+    assert_equal(1, indifferent_strings[:a])
+    assert_equal(1, indifferent_strings["a"])
+    assert_instance_of ActiveSupport::HashWithIndifferentAccess, indifferent_strings
   end
 
   def test_indifferent_transform_values