workbook.name can be "x:workbook" too

[Emil Perhinschi]

at line 820

	assert(workbook.name == "workbook" || workbook.name == "s:workbook");

workbook.name can be "x:workbook" too https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.workbook?view=openxml-2.8.1 , though I'm not sure what the difference is