Ruby implementation of JSON.pretty_generate generates different output to C extension for at least an empty hash

[chrisseaton]

From oracle/truffleruby#2053.

require 'json'
puts JSON.pretty_generate({})

% chruby 2.6.5
% ruby -v test.rb
ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin19]
{
}
% chruby truffleruby-20.1.0
% ruby -v test.rb
truffleruby 20.1.0, like ruby 2.6.5, GraalVM CE Native [x86_64-darwin]
{

}

@eregon then pointed out:

$ ruby -v -e 'require "json/pure"; puts JSON.pretty_generate({})'
ruby 2.6.6p146 (2020-03-31 revision 67876) [x86_64-linux]
{

}

Possibly related to #440?

[chrisseaton]

This is JSON 2.1.0, I think as shipped in MRI 2.6.6.