[examples] Improvement on captcha example

[Christian Grobmeier]

Hi,

the patch below enables the captch example to create different
captcha-challenges.
I think the two lines should go to the invalidate method. If you agree
I can apply a patch for CaptchaImageResource instead of this one.

Best regards,
Christian

Index: src/main/java/org/apache/wicket/examples/captcha/Captcha.java
===================================================================
--- src/main/java/org/apache/wicket/examples/captcha/Captcha.java       
(revision
1063222)
+++ src/main/java/org/apache/wicket/examples/captcha/Captcha.java       
(working copy)
@@ -82,6 +82,8 @@
                        }

                        // force redrawing
+                       imagePass = randomString(6, 8);
+                       
captchaImageResource.getChallengeIdModel().setObject(imagePass);
                        captchaImageResource.invalidate();
                }
        }
@@ -103,7 +105,7 @@
        }

        /** Random captcha password to match against. */
-       private final String imagePass = randomString(6, 8);
+       private String imagePass = randomString(6, 8);

        private final ValueMap properties = new ValueMap();