Check size in bytes as opposed to string size

size returns length in characters, but doesn't factor in multibyte Unicode characters.
By switching to bytesize, we check the relevant measure of how many bytes the worksheet name is.

Fixes randym#588.

lib/axlsx/workbook/worksheet/worksheet.rb

@@ -656,7 +656,7 @@ def outline(collection, range, level = 1, collapsed = true)
 
     def validate_sheet_name(name)
       DataTypeValidator.validate :worksheet_name, String, name
-      raise ArgumentError, (ERR_SHEET_NAME_TOO_LONG % name) if name.size > 31
+      raise ArgumentError, (ERR_SHEET_NAME_TOO_LONG % name) if name.bytesize > 31
       raise ArgumentError, (ERR_SHEET_NAME_CHARACTER_FORBIDDEN % name) if '[]*/\?:'.chars.any? { |char| name.include? char }
       name = Axlsx::coder.encode(name)
       sheet_names = @workbook.worksheets.reject { |s| s == self }.map { |s| s.name }