Make ActiveModel::Errors backward compatible with 4.2

If a Error object was serialized in the database as YAML in the Rails
4.2 version, if we load in the Rails 5.0 version it will miss the
@details instance variable so methods like #clear and #add will start to
fail.

activemodel/lib/active_model/errors.rb

@@ -419,16 +419,23 @@ module ActiveModel
       I18n.translate(key, options)
     end
 
-    def marshal_dump
+    def marshal_dump # :nodoc:
       [@base, without_default_proc(@messages), without_default_proc(@details)]
     end
 
-    def marshal_load(array)
+    def marshal_load(array) # :nodoc:
       @base, @messages, @details = array
       apply_default_array(@messages)
       apply_default_array(@details)
     end
 
+    def init_with(coder) # :nodoc:
+      coder.map.each { |k, v| instance_variable_set(:"@#{k}", v) }
+      @details ||= {}
+      apply_default_array(@messages)
+      apply_default_array(@details)
+    end
+
   private
     def normalize_message(attribute, message, options)
       case message

activemodel/test/cases/errors_test.rb

@@ -365,4 +365,24 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal errors.messages, serialized.messages
     assert_equal errors.details, serialized.details
   end
+
+  test "errors are backward compatible with the Rails 4.2 format" do
+    yaml = <<-CODE.strip_heredoc
+    --- !ruby/object:ActiveModel::Errors
+    base: &1 !ruby/object:ErrorsTest::Person
+      errors: !ruby/object:ActiveModel::Errors
+        base: *1
+        messages: {}
+    messages: {}
+    CODE
+
+    errors = YAML.load(yaml)
+    errors.add(:name, :invalid)
+    assert_equal({ name: ["is invalid"] }, errors.messages)
+    assert_equal({ name: [{ error: :invalid }] }, errors.details)
+
+    errors.clear
+    assert_equal({}, errors.messages)
+    assert_equal({}, errors.details)
+  end
 end