udpated xss_clean() in the security helper to pass $is_image instead of the deprecated $charset

http://codeigniter.com/bug_tracker/bug/6706/
diff --git a/user_guide/changelog.html b/user_guide/changelog.html
index 1992ebb..7a26cb3 100644
--- a/user_guide/changelog.html
+++ b/user_guide/changelog.html
@@ -127,6 +127,7 @@
 	<li>Fixed a bug in the Text Helper character limiter where the provided limit intersects the last word of the string.</li>
 	<li>Fixed a bug (#6342) with plural() in the Inflection helper with words ending in "y".</li>
 	<li>Fixed bug (#6517) where Routed URI segments returned by URI::rsegment() method were incorrect for the default controller.</li>
+	<li>Fixed a bug (#6706) in the Security Helper where xss_clean() was using a deprecated second argument.</li>
 	<li>Tweaked Typography::auto_typography() for some edge-cases.</li>
 </ul>