format key might not right, so need check if the key is exist

[liuguodong15]

                            /*
                                func (c *NumberCol) String(wb *WorkBook) []string {
                                    if fNo := wb.Xfs[c.Index].formatNo(); fNo != 0 {
                                        t := timeFromExcelTime(c.Float, wb.dateMode == 1)
                                        if value,ok:=wb.Formats[fNo];ok{
                                            return []string{yymmdd.Format(t, value.str)}
                                        }else {
                                            return []string{strconv.FormatFloat(c.Float, 'f', -1, 64)}
                                        }
                                    }
                                    return []string{strconv.FormatFloat(c.Float, 'f', -1, 64)}
                                }
                            */
                            col := row.Col(j) //has something wrong, need change above function in file col.go

[liuguodong15]