fix(diff): retain undefined and NaN in output

[lukeed]

By default, JSON.stringify hides undefined and NaN values.

This can be super annoying when your assertion fails correctly, but doesn't show you the reason(s) why. Take these comparisons for example:

You can see that direct, simple comparisons (via assert.is) are fine. But any instances within complex values (object, arrays) were lost. AKA, the { foo: NaN } was printed as { foo: null } -- hence grey text. We see this again with the last assertion with { foo: null } ... and then { foo: undefined } is hidden entirely, leaving an empty Expected: block.

After these changes, undefined and NaN are printed in the diff'ing output so that we have true data representations~!

[codecov-commenter]

Codecov Report

Merging #42 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #42   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files           3        3           
  Lines         302      302           
=======================================
  Hits          272      272           
  Misses         30       30           

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