Fix inconsistencies writing credentials values

Using [] or the dynamic accessors don't result in the same value because
`[]` is delegated to `config` (the decrypted deserialized YAML), whereas
`[]=` and the dynamic accessors are delegated to `options`, an
ActiveSupport::OrderedOptions instance.

activesupport/lib/active_support/encrypted_configuration.rb

@@ -4,6 +4,7 @@
 require "active_support/encrypted_file"
 require "active_support/ordered_options"
 require "active_support/core_ext/object/inclusion"
+require "active_support/core_ext/hash/keys"
 require "active_support/core_ext/module/delegation"
 
 module ActiveSupport
@@ -42,7 +43,6 @@ def message
       end
     end
 
-    delegate :[], :fetch, to: :config
     delegate_missing_to :options
 
     def initialize(config_path:, key_path:, env_key:, raise_if_missing_key:)
@@ -84,7 +84,7 @@ def inspect # :nodoc:
       def deep_transform(hash)
         return hash unless hash.is_a?(Hash)
 
-        h = ActiveSupport::InheritableOptions.new
+        h = ActiveSupport::OrderedOptions.new
         hash.each do |k, v|
           h[k] = deep_transform(v)
         end

activesupport/test/encrypted_configuration_test.rb

@@ -56,6 +56,16 @@ class EncryptedConfigurationTest < ActiveSupport::TestCase
     assert_equal @credentials.config, {}
   end
 
+  test "writing with element assignment and reading with element reference" do
+    @credentials[:foo] = 42
+    assert_equal 42, @credentials[:foo]
+  end
+
+  test "writing with dynamic accessor and reading with element reference" do
+    @credentials.foo = 42
+    assert_equal 42, @credentials[:foo]
+  end
+
   test "change configuration by key file" do
     @credentials.write({ something: { good: true } }.to_yaml)
     @credentials.change do |config_file|