make the proper test to be able to call un/cache-image

closes PR 11598

collects/2htdp/private/universe-image.rkt

@@ -1,7 +1,8 @@
-#lang scheme/base
+#lang racket/base
 (require (prefix-in 2: 2htdp/image)
          (prefix-in 2-core: mrlib/image-core)
          (prefix-in 1: htdp/image)
+         racket/class
          htdp/error)
 
 (provide image? scene? image-width image-height text 2:image?
@@ -9,7 +10,7 @@
          disable-cache)
 
 (define (disable-cache x)
-  (if (2:image? x)
+  (if (is-a? x 2-core:image%)
       (2-core:un/cache-image x #f)
       x))