Unfreeze string literals for ParseNode#inspect

[jalyna]

Since newrelic is using the #inspect method on ParseNode the ruby code crashes due to frozen strings in inspect, which are recursively called with gsub! causing this error:

I added a regression test that fails with the current code.

[amatsuda]

@jalyna Thank you for the patch!
I understand that we need the first children.each_with_object(''.dup) modification, but I don't see why we need the second .dup call at the end of that line.

[jalyna]

@amatsuda this is a really good question! the second .dup is still needed because %Q[] creates a frozen string literal (still). The test will fail if it gets removed.

# frozen_string_literal: true

puts %Q[hello world].frozen?

prints true.

We could also use something else here 🤔

[amatsuda]

@jalyna Ah! I just got what you meant by "which are recursively called". Thank you for a clear explanation (and I'm sorry that I wasn't carefully reading it...)!

[amatsuda]

@jalyna Just published version 5.1.2 gem with this patch. Thanks!