Fix caching issue happening with memory_store

It seems that fecthing from memory_store returns a reference to the
object and not a copy. Since the Attributes adapter applies #merge! on
the Hash that is returned from the memory_store, the value in the cache
is also modified.

lib/active_model_serializers/adapter/attributes.rb

@@ -55,7 +55,7 @@ def cached_attributes(cached_serializer)
       def serializable_hash_for_single_resource(options)
         resource = resource_object_for(options)
         relationships = resource_relationships(options)
-        resource.merge!(relationships)
+        resource.merge(relationships)
       end
 
       def resource_relationships(options)

test/support/rails_app.rb

@@ -8,7 +8,7 @@ module ActiveModelSerializers
       # TODO: figure out why turning on the memory cache changes
       # the result of the CacheTest#test_associations_cache_when_updated
       # and if it is more correct or less correct.
-      # config.action_controller.cache_store = :memory
+      config.action_controller.cache_store = :memory_store
     end
 
     app.routes.default_url_options = { host: 'example.com' }