Allow mutations to return custom classes

[IvanGoncharov]

Fixes #339

[codecov]

Codecov Report

Merging #349 (1c3f699) into main (fa978fb) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #349   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines          732       745   +13     
  Branches        50        51    +1     
=========================================
+ Hits           732       745   +13     

Impacted Files	Coverage Δ
src/mutation/__tests__/mutation-test.js	100.00% <100.00%> (ø)
src/mutation/mutation.js	100.00% <100.00%> (ø)

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