core: use addCompressionHandler() instead directly adding to the list

smack-core/src/main/java/org/jivesoftware/smack/SmackInitialization.java

@@ -107,7 +107,7 @@ public final class SmackInitialization {
         }
 
         // Add the Java7 compression handler first, since it's preferred
-        SmackConfiguration.compressionHandlers.add(new Java7ZlibInputOutputStream());
+        SmackConfiguration.addCompressionHandler(new Java7ZlibInputOutputStream());
 
         XmppCompressionManager.registerXmppCompressionFactory(ZlibXmppCompressionFactory.INSTANCE);