Unit tests fail in 3.4.0

[suetanvil]

Two unit tests fail when building MRuby 3.4.0 against this gem:

1. There is a syntax error in test/json.rb:148 that is almost certainly due to the parser becoming more correct. The expression in question is JSON.load '{' {|x| o = x}; putting parens around the argument to load fixed the problem.
2. On line 35, the output differs from the expected output only by insignificant whitespace. I'm not sure why this changed, but the result is still correct.

I've fixed this and will submit a pull request.