Merge pull request #2482 from vineelvineel/fix_cant_modify_frozen_hash

Fix cant modify frozen Hash error due to rails changes

lib/active_model_serializers/adapter/attributes.rb

@@ -9,7 +9,7 @@ def initialize(*)
       end
 
       def serializable_hash(options = nil)
-        options = serialization_options(options)
+        options = serialization_options(options.dup)
         options[:fields] ||= instance_options[:fields]
         serialized_hash = serializer.serializable_hash(instance_options, options, self)