When batching, update the 'when' time to the lesser of the existing b…

…atch and the new job.

lib/MongoQueue.php

@@ -42,7 +42,7 @@ public static function push($className, $methodName, $parameters, $when, $batch
 			{
 				$job = $job['value'];
 
-				if (!isset($job['when']))
+				if (!isset($job['when']) || $job['when'] > $when)
 				{
 					$job['when'] = $when;
 					$collection->save($job);