adding a missing 2htdp/image color => racket/gui color coercion

closes PR 11956

collects/2htdp/tests/test-image.rkt

@@ -1665,6 +1665,14 @@
        160 160 0 1/2
        (make-pen "black" 12 "solid" "round" "round")))
 
+(test (add-line (rectangle 30 30 "outline" "black")
+                0 0 30 30 
+                (make-pen (make-color 0 0 0 255) 15 "solid" "butt" "round"))
+      =>
+      (add-line (rectangle 30 30 "outline" "black")
+                0 0 30 30 
+                (make-pen "black" 15 "solid" "butt" "round")))
+
 (test (image->color-list
        (above (beside (rectangle 1 1 'solid (color 1 1 1))
                       (rectangle 1 1 'solid (color 2 2 2))

collects/mrlib/image-core.rkt

@@ -1136,7 +1136,7 @@ the mask bitmap and the original bitmap are all together in a single bytes!
 
 (define (pen->pen-obj/cache pen)
   (send the-pen-list find-or-create-pen 
-        (pen-color pen)
+        (get-color-arg (pen-color pen))
         (pen-width pen)
         (pen-style pen)
         (pen-cap pen)