Add PDO support for list_fields()

Signed-off-by: Andrey Andreev <narf@bofh.bg>
diff --git a/system/database/drivers/pdo/pdo_result.php b/system/database/drivers/pdo/pdo_result.php
index 0b8937c..8671056 100644
--- a/system/database/drivers/pdo/pdo_result.php
+++ b/system/database/drivers/pdo/pdo_result.php
@@ -124,12 +124,14 @@
 	 */
 	public function list_fields()
 	{
-		if ($this->db->db_debug)
+		$field_names = array();
+		for ($i = 0, $c = $this->num_fields(); $i < $c; $i++)
 		{
-			return $this->db->display_error('db_unsuported_feature');
+			$field_names[$i] = @$this->result_id->getColumnMeta();
+			$field_names[$i] = $field_names[$i]['name'];
 		}
 
-		return FALSE;
+		return $field_names;
 	}
 
 	// --------------------------------------------------------------------