Setter is assuming unicode encoding

MySQL encoding does not seem to translate directly to PHP character encoding names. Setter is assuming that value is unicode and might produce unexpected results.

```
if (!is_null(static::$columns[$name]['character_maximum_length']) && static::$columns[$name]['character_maximum_length'] < mb_strlen($value)) {
throw new Exception\InvalidArgumentException('Property does not conform to the column\'s maxiumum character length limit.');
}
```