Merge pull request #1186 from bf4/empty_else

Enforce case requires else; allow else nil

lib/active_model/serializer/include_tree.rb

@@ -102,6 +102,8 @@ module ActiveModel
           self.class.new(@hash[:*])
         when @hash.key?(:**)
           self.class.new(:** => {})
+        else
+          nil
         end
       end
     end