fixed processRenamedColumns() to work with mysql migrations

lib/Doctrine/Migration/Process.php

@@ -116,9 +116,14 @@ class Doctrine_Migration_Process
     public function processRenamedColumns($columns)
     {
         foreach ($columns as $column) {
-            $conn = $this->getConnection($column['tableName']);
-            
-            $conn->export->alterTable($column['tableName'], array('rename' => array($column['oldColumnName'] => array('name' => $column['newColumnName']))));
+            $conn = $this->getConnection($column['tableName']);
+            
+            $columnList = $conn->import->listTableColumns($column['tableName']);
+            if (isset($columnList[$column['oldColumnName']])) {
+	            $conn->export->alterTable($column['tableName'], 
+	                                      array('rename' => array($column['oldColumnName'] => array('name' => $column['newColumnName'],
+	                                      																													'definition'=>$columnList[$column['oldColumnName']]))));
+            }
         }
     }