Fixes isHandling() result not being acknowledged in handle() in AbstractProcessingHandler

When a handler that extends `AbstractProcessingHandler` overrides `isHandling`, the returned value isn't acknowledged within the `handle` method. This causes records to not be written when `isHandling` doesn't necessarily match up with the activation level of the handler.

```php
class MyHandler extends \Monolog\Handler\AbstractProcessingHandler {
    public function isHandling(array $record) {
        return $record['level'] < $this->level || $some_other_condition;
    }
}
```