Overload save and restore to fix a serialization issue

[ablaom]

JuliaAI/MLJ.jl#1099

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.00%. Comparing base (b12c0bc) to head (4db6d59).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #59      +/-   ##
==========================================
+ Coverage   96.93%   97.00%   +0.06%     
==========================================
  Files           7        7              
  Lines         196      200       +4     
==========================================
+ Hits          190      194       +4     
  Misses          6        6              

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[paulotrefosco]

Hello @ablaom ,
Sorry for taking so long to answer, I had to make sure the problem wasn't me. I made some tests and seems to be working properly now (saving and restoring Tuned XGBoost models).
Thanks a lot! Best regards