ZF-2432: Zend_Validate validators do not all set messages
A number of Zend_Validate concrete validators do not currently set the $_messages array on a failed isValid() call, which means that if you call getMessages() after a failed validation, it may return an empty result. This is problematic when using a validator chain (such as Zend_Validate, or the new Zend_Form component), as the results of getMessages() becomes unpredictable.
Validators known to be missing this functionality include: * Digits * Alnum
Additionally, it would be useful to have the returned array use the error code for the key, such that getMessages() would return array( CODE => ).