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]