Fix argument specs to `Fiber`

mrbgems/mruby-fiber/src/fiber.c

@@ -409,7 +409,7 @@ mrb_mruby_fiber_gem_init(mrb_state* mrb)
   c = mrb_define_class(mrb, "Fiber", mrb->object_class);
   MRB_SET_INSTANCE_TT(c, MRB_TT_FIBER);
 
-  mrb_define_method(mrb, c, "initialize", fiber_init,    MRB_ARGS_NONE());
+  mrb_define_method(mrb, c, "initialize", fiber_init,    MRB_ARGS_NONE()|MRB_ARGS_BLOCK());
   mrb_define_method(mrb, c, "resume",     fiber_resume,  MRB_ARGS_ANY());
   mrb_define_method(mrb, c, "transfer",   fiber_transfer, MRB_ARGS_ANY());
   mrb_define_method(mrb, c, "alive?",     fiber_alive_p, MRB_ARGS_NONE());