Ensure mxRecord is an array
Ensure mxRecord is an array

- `getmxrr` can potentially nullify its `$mxhosts` argument.
- `array_combine`, on failure, returns boolean `false`.

The changes made ensure that `$mxRecord` is an array after attempting to
retrieve MX records, to ensure tests pass in HHVM.

ezimuel/zend-validator,svycka/zend-validator,akrabat/zend-validator,zf2timo/zend-validator,zendframework/zend-validator