Differences in jruby and mri json output

I often have the problem that there are minor differences between the output of an approval test in jruby and mri. We run our tests in both rubies so the approval test always fails for such cases in one of the rubies.

An example of this is that the following generates different approvals in jruby and mri:

verify(format: :json) { { weight: {} } }

In jruby:

weight: {

}

in mri:

weight: {
}

Is there any way to avoid this?

The problem seems to be jsons pretty_generate: https://github.com/kytrinyx/approvals/blob/master/lib/approvals/writers/json_writer.rb#L11

jruby:

JSON.pretty_generate({ weight: {} })
=> "{\n  \"weight\": {\n\n  }\n}"`

mri:

JSON.pretty_generate({ weight: {} })
=> "{\n  \"weight\": {\n  }\n}"

Start in lib/approvals/writers/json_writer.rb at the JSON.pretty_generate call, then reproduce the shown output on MRI and JRuby. Done means approval output for the same JSON input is consistent across both Ruby runtimes; the issue does not name a test file.