Allow attributes with a proc default to be marshalled

We don't implement much custom marshalling logic for these objects, but
the proc default case needs to be handled separately. Unfortunately
there's no way to just say "do what you would have done but with this
value for one ivar", so we have to manually implement `marshal_load` as
well.

The test case is a little bit funky, but I'd really like an equality
test in there, and there's no easy way to add one now that this is out
of AR (since the `attributes` method isn't here)

Fixes #31216

activemodel/CHANGELOG.md

+*   Models using the attributes API with a proc default can now be marshalled.
+
+    Fixes #31216.
+
+    *Sean Griffin*
+
 *   Fix to working before/after validation callbacks on multiple contexts.
 
     *Yoshiyuki Hirano*
 
+
 ## Rails 5.2.0.beta2 (November 28, 2017) ##
 
 *   No changes.

activemodel/lib/active_model/attribute/user_provided_default.rb

@@ -22,6 +22,28 @@ def with_type(type)
         self.class.new(name, user_provided_value, type, original_attribute)
       end
 
+      def marshal_dump
+        result = [
+          name,
+          value_before_type_cast,
+          type,
+          original_attribute,
+        ]
+        result << value if defined?(@value)
+        result
+      end
+
+      def marshal_load(values)
+        name, user_provided_value, type, original_attribute, value = values
+        @name = name
+        @user_provided_value = user_provided_value
+        @type = type
+        @original_attribute = original_attribute
+        if values.length == 5
+          @value = value
+        end
+      end
+
       protected
 
         attr_reader :user_provided_value

activemodel/test/cases/attributes_test.rb

@@ -64,5 +64,14 @@ class GrandchildModelForAttributesTest < ChildModelForAttributesTest
 
       assert_equal "4.4", data.integer_field
     end
+
+    test "attributes with proc defaults can be marshalled" do
+      data = ModelForAttributesTest.new
+      attributes = data.instance_variable_get(:@attributes)
+      round_tripped = Marshal.load(Marshal.dump(data))
+      new_attributes = round_tripped.instance_variable_get(:@attributes)
+
+      assert_equal attributes, new_attributes
+    end
   end
 end