Use reflection key since we have it

Author: bf4

lib/active_model/serializer.rb

@@ -337,9 +337,8 @@ def associations(include_directive = ActiveModelSerializers.default_include_dire
       return Enumerator.new unless object
 
       Enumerator.new do |y|
-        self.class._reflections.values.each do |reflection|
+        self.class._reflections.each do |key, reflection|
           next if reflection.excluded?(self)
-          key = reflection.options.fetch(:key, reflection.name)
           next unless include_directive.key?(key)
 
           y.yield reflection.build_association(self, instance_options, include_slice)