fix identity_map primary key checking

lib/sequel/plugins/identity_map.rb

@@ -123,7 +123,7 @@ def _associated_object_pk(fk)
         # map for the associated object and return it if present.
         def _load_associated_objects(opts)
           klass = opts.associated_class
-          if klass.respond_to?(:identity_map) && idm = klass.identity_map and opts[:type] == :many_to_one and opts[:primary_key] == klass.primary_key and
+          if klass.respond_to?(:identity_map) && idm = klass.identity_map and opts[:type] == :many_to_one and opts.primary_key == klass.primary_key and
            opts[:key] and pk = _associated_object_pk(opts[:key]) and o = idm[klass.identity_map_key(pk)]
             o
           else