Add tests of json_dict() method

[jwodder]

This PR adds tests of the json_dict() method to test_metadata.py so that we can be sure it generates what we expect and won't drop schemaKeys again.

This PR includes #525.

[codecov]

Codecov Report

Merging #526 (90d24dd) into master (a6a4a0f) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #526      +/-   ##
==========================================
+ Coverage   83.49%   83.52%   +0.02%     
==========================================
  Files          62       62              
  Lines        6418     6428      +10     
==========================================
+ Hits         5359     5369      +10     
  Misses       1059     1059              

Flag	Coverage Δ
unittests	83.52% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
dandi/tests/test_metadata.py	100.00% <100.00%> (ø)

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