Add support for configurable NULL encoding about text-csv_xs

[Tux]

Sorry, but you did not mention one critical issue that causes the behavior you see, as in the default setup, it works exactly as documented:

$ perl -MText::CSV_XS -e'Text::CSV_XS->new->say(*STDOUT,[map{$_//"\\N"}"blah",undef,3])'
blah,\N,3

But I am sure you have setup your instance with { escape => "\\" }, which causes the need to escape the escape, and fields that have escapes in them are automatically quoted:

$ perl -MText::CSV_XS -e'Text::CSV_XS->new({escape=>"\\"})->say(*STDOUT,[map{$_//"\\N"}"blah",undef,3])'
blah,"\\N",3

I'll see if there can be a more explicit attribute to achieve this, as callbacks are not an option to make this combination work.

[Tux]

Try a pull/clone from here. I added the undef_str attribute:

$ perl -Mblib -MText::CSV_XS -e'Text::CSV_XS->new({escape=>"\\",undef_str=>"\\N"})->say(*STDOUT,["blah",undef,3])'perl -Mblib -MText::CSV_XS -e'Text::CSV_XS->new({escape=>"\\",undef_str=>"\\N"})->say(*STDOUT,["blah",undef,3])'
blah,\N,3

See https://github.com/Tux/Text-CSV_XS/blob/master/doc/CSV_XS.md#undef_str

[dankasak]

Wow, that's great! Thankyou so much for the fast response and patch.

[Tux]

Feedback would be more than welcome BTW.
I saw it failed tests on 5.18.x and below, so I cannot release yet.
(and I just pushed a patch that allows UTF-8 values for undef_str, where ∅ (U+002205) springs to mind as useful :)

[Tux]

@dankasak would you be so kind to pull/clone again? I changed a lot of code to make it work on 5.18.x and below. If my tests do not cover all issues, your code might.
Any other feedback for now?

[dankasak]

I've pulled and re-tested, using a couple of options for supporting different databases. Looks good to me :)

[Tux]

Thanks for the feedback. I'll start the big test-to-release process