fix(parse): default bare JSON.Object keys to String

[quinnj]

Summary

• make JSON.parse(json, JSON.Object) materialize JSON.Object{String, Any}
• add a regression test for issue JSON.parse(json, T) keys to Any instead of String #456

Testing

• julia --project=. -e 'using Pkg; Pkg.test(; coverage=false)'

Fixes #456

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.25%. Comparing base (1efd912) to head (30f6534).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #457   +/-   ##
=======================================
  Coverage   90.25%   90.25%           
=======================================
  Files           7        7           
  Lines        1395     1396    +1     
=======================================
+ Hits         1259     1260    +1     
  Misses        136      136           

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