Do not have Stanza.getExtension(String, String) return a generic type

Returning a generic would allow for

List<ExtensionElement> list = stanza.getExtension("foo", "bar");

to compile (Note the we are calling getExtension(), not
getExtension*s*()).

Users are encouraged to use the type safe getExtension(Class<? extends
ExtensionElement) variant instead.

Fixes SMACK-825.

smack-extensions/src/main/java/org/jivesoftware/smackx/offline/OfflineMessageManager.java

@@ -159,8 +159,7 @@ public class OfflineMessageManager {
         StanzaFilter messageFilter = new AndFilter(PACKET_FILTER, new StanzaFilter() {
             @Override
             public boolean accept(Stanza packet) {
-                OfflineMessageInfo info = packet.getExtension("offline",
-                        namespace);
+                OfflineMessageInfo info = packet.getExtension(OfflineMessageInfo.class);
                 return nodes.contains(info.getNode());
             }
         });