Fix thread unsafe behavior

Author: LongCB

lib/active_model/serializer.rb

@@ -347,7 +347,7 @@ def associations(include_directive = ActiveModelSerializers.default_include_dire
       return Enumerator.new {} unless object
 
       Enumerator.new do |y|
-        self.class._reflections.each do |key, reflection|
+        (@reflections ||= self.class._reflections.deep_dup).each do |key, reflection|
           next if reflection.excluded?(self)
           next unless include_directive.key?(key)
 
@@ -411,6 +411,6 @@ def associations_hash(adapter_options, options, adapter_instance)
 
     protected
 
-    attr_accessor :instance_options
+    attr_accessor :instance_options, :reflections
   end
 end