Copied sheet mirrors old one

[the-goodies]

All copied sheets reference the same underlying one. So modifying any sheet, it applies to all of them.

func sampleCopySheet() {
	wb := spreadsheet.New()
	defer wb.SaveToFile("error.xlsx")

	sheet := wb.AddSheet()
	sheet.Cell("A1").SetString("one")

	sheet, _ = wb.CopySheetByName("Sheet 1", "Sheet 2")
	sheet.Cell("A1").SetString("two")

	sheet, _ = wb.CopySheetByName("Sheet 1", "Sheet 3")
	sheet.Cell("A1").SetString("three")
}

Correct result: "Sheet 1:A1" - "one"; "Sheet 2:A1" - "two" ; "Sheet 3:A1" - "three".
How it is now: "Sheet 1:A1" - "three"; "Sheet 2:A1" - "three" ; "Sheet 3:A1" - "three".

[gunnsth]

Looks like this requires a deep copy of the sheet contents. Maybe a new function called DeepCopySheetByName to keep compatibility.