Low-order non-printable ASCII characters in worksheet name should be escaped or removed

[khiav223577]

The worksheet name was escaped as HTML since https://github.com/randym/axlsx/issues/112. But It seems to be not working with some special characters, "\b" character for example. I think it should be escaped as XML.

See: https://support.microsoft.com/en-us/help/315580/prb-error-message-when-an-xml-document-contains-low-order-ascii-charac

The following are the character ranges for low-order non-printable ASCII characters that are rejected by MSXML versions 3.0 and later:

x0 - #x8 (ASCII 0 - 8)

xB - #xC (ASCII 11 - 12)

xE - #x1F (ASCII 14 - 31)

Steps to reproduce

1. Export an excel file by aslsx.

   
   require 'axlsx'

Axlsx::Package.new do |p| p.workbook.add_worksheet(:name => "My sheet\b") do |sheet| sheet.add_row ["Data1", "Data2"] end p.serialize('simple1.xlsx') end

2. Double click `simple1.xlsx` to open it.

It will say that the excel is corrupted.
![2020-08-24_154115](https://user-images.githubusercontent.com/4011729/91017145-68370680-e620-11ea-97b9-3865bf30c9ab.png)

### Possible fixes

We can use `Builder::XChar.encode` to escape the worksheet name.

```rb
require 'axlsx'

Axlsx::Package.new do |p|
  p.workbook.add_worksheet(:name => Builder::XChar.encode("My sheet\b")) do |sheet|
    sheet.add_row ["Data1", "Data2"]
  end
  p.serialize('simple2.xlsx')
end

The produced excel file simple2.xlsx works fine.

[noniq]

Does it make sense to retain such (escaped) characters in sheet names? In cells for example they are removed, see Cell#clean_value which calls Axlsx.sanitize which removes control characters.

[khiav223577]

@noniq I'm not sure which is better, removing or escaping non-printable ASCII characters in sheet names. If it is undocumented or is undefined behavior, it's best to be removed. if it is a valid name, it make sense to retain it.

[noniq]

My gut feeling is that non-printable characters should not be allowed to be part of a sheet name. Are there any references to what constitutes a valid sheet name in the Excel docs?

[khiav223577]

I didn't find any references about what to do with non-printable characters :thinking:. Either removing or escaping is okay to me.

[noniq]

Would you like to prepare a pull request? axlsx is currently unmaintained though, so please create the pull request in the https://github.com/caxlsx/caxlsx repository (that's the actively maintained community fork).