Mix ActiveModel::AttributeAssignment into Base

By including the [ActiveModel::AttributeAssignment][], the `Base` class
can access the [assign_attributes][] method for bulk assignment of
attributes **without** saving them to the server (like through
`Base#update_attributes`).

```ruby
Person.schema = { name: "string" }

person = Person.new
person.id   # => nil
person.name # => nil

person.assign_attributes id: 1, name: "Matz"
person.id   # => 1
person.name # => "Matz"
```
[ActiveModel::AttributeAssignment]: https://edgeapi.rubyonrails.org/classes/ActiveModel/AttributeAssignment.html
[assign_attributes]: https://edgeapi.rubyonrails.org/classes/ActiveModel/AttributeAssignment.html#method-i-assign_attributes

Author: seanpdoyle

lib/active_resource/base.rb

@@ -1850,7 +1850,7 @@ class Base
     extend ActiveResource::Associations
 
     include Callbacks, CustomMethods, Validations, Serialization
-    include ActiveModel::Conversion
+    include ActiveModel::Conversion, ActiveModel::AttributeAssignment
     include ActiveModel::ForbiddenAttributesProtection
     include ActiveModel::Serializers::JSON
     include ActiveModel::Serializers::Xml

test/cases/base_test.rb

@@ -1008,6 +1008,28 @@ def test_predicate_for_an_unknown_attribute_returns_nil
     assert_not_predicate post, :unknown_attribute?
   end
 
+  def test_assign_attributes_to_known_attributes
+    previous_schema = Person.schema
+    Person.schema = { name: "string" }
+
+    matz = Person.new
+
+    matz.assign_attributes "id" => 1, :name => "Matz"
+
+    assert_equal 1, matz.id
+    assert_equal "Matz", matz.name
+  ensure
+    Person.schema = previous_schema
+  end
+
+  def test_assign_attributes_assigns_to_loaded_attributes
+    matz = Person.new(name: "matz")
+
+    assert_changes -> { matz.name }, from: "matz", to: "Matz" do
+      matz.assign_attributes(name: "Matz")
+    end
+  end
+
   def test_custom_header
     Person.headers["key"] = "value"
     assert_raise(ActiveResource::ResourceNotFound) { Person.find(4) }