Fix has_many association _ids helper for Mongoid 3

lib/aasm/persistence/mongoid_persistence.rb

@@ -99,8 +99,11 @@ def aasm_ensure_initial_state
             attribute_name = self.class.aasm(state_machine_name).attribute_name.to_s
             # Do not load initial state when object attributes are not loaded,
             # mongoid has_many relationship does not load child object attributes when
-            # only ids are loaded, for example parent.child_ids will not load child object.
-            if attribute_names.include?(attribute_name) && attributes[attribute_name].blank? && !attribute_missing?(attribute_name)
+            # only ids are loaded, for example parent.child_ids will not load child object attributes.
+            # This feature is introduced in mongoid > 4.
+            if attribute_names.include?(attribute_name) && attributes[attribute_name].blank?
+              # attribute_missing? is defined in mongoid > 4
+              return if Mongoid::VERSION.to_f >= 4 && attribute_missing?(attribute_name)
               send("#{self.class.aasm(state_machine_name).attribute_name}=", aasm(state_machine_name).enter_initial_state.to_s)
             end
           end