Ensure that he returned extension element is actually of the correct
type. It should not be common, but in theory, Smack's provider
architecture would allow for different types to be returned than the
ones one may expect.
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.