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/xdata/packet/DataForm.java

@@ -361,7 +361,7 @@ public class DataForm implements ExtensionElement {
      * @return the DataForm or null
      */
     public static DataForm from(StanzaView stanzaView) {
-        return (DataForm) stanzaView.getExtension(QNAME);
+        return stanzaView.getExtension(DataForm.class);
     }
 
     /**