Adding Fragment Cache to AMS

It's an upgrade based on the new Cache implementation #693.
It allows to use the Rails conventions to cache
specific attributes or associations.
It's based on the Cache Composition implementation.

test/adapter/fragment_cache_test.rb

@@ -0,0 +1,27 @@
+require 'test_helper'
+module ActiveModel
+  class Serializer
+    class Adapter
+      class FragmentCacheTest < Minitest::Test
+        def setup
+          @author          = Author.new(name: 'Joao M. D. Moura')
+          @role            = Role.new(name: 'Great Author', description:nil)
+          @role.author     = [@author]
+          @role_serializer = RoleSerializer.new(@role)
+          @role_hash       = FragmentCache.new(RoleSerializer.adapter.new(@role_serializer), @role_serializer, {}, nil)
+        end
+
+        def test_fragment_fetch_with_virtual_attributes
+          expected_result = {
+            id: @role.id,
+            description: @role.description,
+            slug: "#{@role.name}-#{@role.id}",
+            name: @role.name
+          }
+          assert_equal(@role_hash.fetch, expected_result)
+        end
+      end
+    end
+  end
+end
+