Cast LOG_DEBUG to string in sample (#340)
Cast LOG_DEBUG to string in sample (#340)

All configuration parameters passed to librdkafka should be strings.
Failure to do so results in:
```
TypeError : RdKafka\Conf::set() expects parameter 2 to be string, integer given
```
arnaud-lb/php-rdkafka,arnaud-lb/php-rdkafka,arnaud-lb/php-rdkafka,arnaud-lb/php-rdkafka