fix 'no LastInsertId available' when mock UPDATE using gorm

[qiulimao]

//return driver.RowsAffected(fResp.RowsAffected), nil
/*******************************************
* the code in jinzhu/scope line 360~374    *
********************************************
/ Exec perform generated SQL
func (scope *Scope) Exec() *Scope {
    defer scope.trace(NowFunc())

    if !scope.HasError() {
        if result, err := scope.SQLDB().Exec(scope.SQL, scope.SQLVars...); scope.Err(err) == nil {

            if count, err := result.RowsAffected(); scope.Err(err) == nil {
                scope.db.RowsAffected = count
            }
            if lastInsertId, err := result.LastInsertId(); scope.Err(err) == nil {
                scope.db.LastInsertId = lastInsertId
            }
        }
    }
    return scope
}
// ********************************************
// here we can see that,if the sql.Result has any error in getting RowsAffected() or
// LastInsertId() the gorm will result an error.
// driver.RowsAffected's  LastInsertId() method will get error.That would case "no LastInsertId available"
// error tips,when mock UPDATE
    */
    // SO: let the user decide when get error,and how many rowsAffected and lastInsetID