fix: copy caller-supplied defaults instead of aliasing

[maxrjones]

This PR isolates the bug fix that @djhoese recommended in #149 (comment) into a distinct PR. I assumed the test that aliased the callers argument was correct; in fact that was a bug in both the code and tests. This PR makes sure that the caller supplied defaults are always copied.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.44%. Comparing base (7b90e4d) to head (a92210b).
⚠️ Report is 83 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
+ Coverage   98.28%   98.44%   +0.15%     
==========================================
  Files           6        6              
  Lines         759      772      +13     
==========================================
+ Hits          746      760      +14     
+ Misses         13       12       -1     

Flag	Coverage Δ
unittests	98.44% <100.00%> (+0.15%)	⬆️

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[djhoese]

Thanks!