Include adapter in cache key

Confirm caching attributes with different key json_api vs. attributes adapter

Adapted from @kevintyll's original test
https://github.com/rails-api/active_model_serializers/pull/1644#issuecomment-204147094

lib/active_model/serializer/caching.rb

@@ -58,6 +58,10 @@ module ActiveModel
           ''.freeze
         end
 
+        def _skip_digest?
+          _cache_options && _cache_options[:skip_digest]
+        end
+
         # @api private
         # Used by FragmentCache on the CachedSerializer
         #  to call attribute methods on the fragmented cached serializer.