Unset $db in DboSource::read().

After calling $db->queryAssociatin(), it is necessary to unset db. 
This reverts commit 52d4257.

lib/Cake/Model/Datasource/DboSource.php

@@ -1140,6 +1140,7 @@ public function read(Model $Model, $queryData = array(), $recursive = null) {
 						$stack['_joined'] = $joined;
 
 						$db->queryAssociation($Model, $LinkModel, $type, $assoc, $assocData, $array, true, $resultSet, $Model->recursive - 1, $stack);
+						unset($db);
 
 						if ($type === 'hasMany' || $type === 'hasAndBelongsToMany') {
 							$filtered[] = $assoc;