fix: RenameIndex should drop old index (go-gorm#151)

Author: black-06

migrator.go

@@ -322,6 +322,9 @@ func (m Migrator) RenameIndex(value interface{}, oldName, newName string) error
 		var sql string
 		m.DB.Raw("SELECT sql FROM sqlite_master WHERE type = ? AND tbl_name = ? AND name = ?", "index", stmt.Table, oldName).Row().Scan(&sql)
 		if sql != "" {
+			if err := m.DropIndex(value, oldName); err != nil {
+				return err
+			}
 			return m.DB.Exec(strings.Replace(sql, oldName, newName, 1)).Error
 		}
 		return fmt.Errorf("failed to find index with name %v", oldName)