Start bringing attributes API to AM

This is the first PR of a WIP to bring the attributes API to
ActiveModel.  It is not yet ready for public API.

The `attributes_dirty_test.rb` file was created based on `dirty_test.rb`,
and the simplifications in the diff do much to motivate this change.

```
diff activemodel/test/cases/dirty_test.rb activemodel/test/cases/attributes_dirty_test.rb
3a4
> require "active_model/attributes"
5c6
< class DirtyTest < ActiveModel::TestCase
---
> class AttributesDirtyTest < ActiveModel::TestCase
7,41c8,12
<     include ActiveModel::Dirty
<     define_attribute_methods :name, :color, :size
<
<     def initialize
<       @name = nil
<       @color = nil
<       @size = nil
<     end
<
<     def name
<       @name
<     end
<
<     def name=(val)
<       name_will_change!
<       @name = val
<     end
<
<     def color
<       @color
<     end
<
<     def color=(val)
<       color_will_change! unless val == @color
<       @color = val
<     end
<
<     def size
<       @size
<     end
<
<     def size=(val)
<       attribute_will_change!(:size) unless val == @size
<       @size = val
<     end
---
>     include ActiveModel::Model
>     include ActiveModel::Attributes
>     attribute :name, :string
>     attribute :color, :string
>     attribute :size, :integer
```

1 files changed, 94 insertions, 0 deletions

diff --git a/activemodel/test/cases/attributes_test.rb b/activemodel/test/cases/attributes_test.rb
new file mode 100644
index 0000000000..064cba40e3
--- /dev/null
+++ b/activemodel/test/cases/attributes_test.rb
@@ -0,0 +1,94 @@
+# frozen_string_literal: true
+
+require "cases/helper"
+require "active_model/attributes"
+
+module ActiveModel
+  class AttributesTest < ActiveModel::TestCase
+    class ModelForAttributesTest
+      include ActiveModel::Model
+      include ActiveModel::Attributes
+
+      attribute :integer_field, :integer
+      attribute :string_field, :string
+      attribute :decimal_field, :decimal
+      attribute :string_with_default, :string, default: "default string"
+      attribute :date_field, :string, default: -> { Date.new(2016, 1, 1) }
+      attribute :boolean_field, :boolean
+    end
+
+    class ChildModelForAttributesTest < ModelForAttributesTest
+    end
+
+    class GrandchildModelForAttributesTest < ChildModelForAttributesTest
+      attribute :integer_field, :string
+    end
+
+    test "properties assignment" do
+      data = ModelForAttributesTest.new(
+        integer_field: "2.3",
+        string_field: "Rails FTW",
+        decimal_field: "12.3",
+        boolean_field: "0"
+      )
+
+      assert_equal 2, data.integer_field
+      assert_equal "Rails FTW", data.string_field
+      assert_equal BigDecimal.new("12.3"), data.decimal_field
+      assert_equal "default string", data.string_with_default
+      assert_equal Date.new(2016, 1, 1), data.date_field
+      assert_equal false, data.boolean_field
+
+      data.integer_field = 10
+      data.string_with_default = nil
+      data.boolean_field = "1"
+
+      assert_equal 10, data.integer_field
+      assert_nil data.string_with_default
+      assert_equal true, data.boolean_field
+    end
+
+    test "dirty" do
+      data = ModelForAttributesTest.new(
+        integer_field: "2.3",
+        string_field: "Rails FTW",
+        decimal_field: "12.3",
+        boolean_field: "0"
+      )
+
+      assert_equal false, data.changed?
+
+      data.integer_field = "2.1"
+
+      assert_equal false, data.changed?
+
+      data.string_with_default = "default string"
+
+      assert_equal false, data.changed?
+
+      data.integer_field = "5.1"
+
+      assert_equal true, data.changed?
+      assert_equal true, data.integer_field_changed?
+      assert_equal({ "integer_field" => [2, 5] }, data.changes)
+    end
+
+    test "nonexistent attribute" do
+      assert_raise ActiveModel::UnknownAttributeError do
+        ModelForAttributesTest.new(nonexistent: "nonexistent")
+      end
+    end
+
+    test "children inherit attributes" do
+      data = ChildModelForAttributesTest.new(integer_field: "4.4")
+
+      assert_equal 4, data.integer_field
+    end
+
+    test "children can override parents" do
+      data = GrandchildModelForAttributesTest.new(integer_field: "4.4")
+
+      assert_equal "4.4", data.integer_field
+    end
+  end
+end