Fix serialization of System.frequency

[daniel-thom]

Fixes #1037

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (073ae04) 85.35% compared to head (eb2f6ea) 85.35%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1038   +/-   ##
=======================================
  Coverage   85.35%   85.35%           
=======================================
  Files         170      170           
  Lines        7691     7691           
=======================================
  Hits         6565     6565           
  Misses       1126     1126           

Flag	Coverage Δ
unittests	85.35% <100.00%> (ø)

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Files	Coverage Δ
src/base.jl	92.25% <100.00%> (ø)