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/xhtmlim/packet/XHTMLExtension.java

@@ -130,6 +130,6 @@ public class XHTMLExtension implements ExtensionElement {
     }
 
     public static XHTMLExtension from(MessageView message) {
-        return message.getExtension(QNAME);
+        return message.getExtension(XHTMLExtension.class);
     }
 }