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.

lib/active_model/serializer/adapter/json_api/fragment_cache.rb

@@ -0,0 +1,22 @@
+module ActiveModel
+  class Serializer
+    class Adapter
+      class JsonApi < Adapter
+        class FragmentCache
+
+          def fragment_cache(root, cached_hash, non_cached_hash)
+            hash              = {}
+            core_cached       = cached_hash.first
+            core_non_cached   = non_cached_hash.first
+            no_root_cache     = cached_hash.delete_if {|key, value| key == core_cached[0] }
+            no_root_non_cache = non_cached_hash.delete_if {|key, value| key == core_non_cached[0] }
+            cached_resource   = (core_cached[1]) ? core_cached[1].merge(core_non_cached[1]) : core_non_cached[1]
+            hash = (root) ? { root => cached_resource } : cached_resource
+            hash.merge no_root_non_cache.merge no_root_cache
+          end
+
+        end
+      end
+    end
+  end
+end