Fix MLJ Serialization, add test for single class classifiers, fix single class classifiers predict

[tylerjthomas9]

Ref #34

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 30 lines in your changes are missing coverage. Please review.

Project coverage is 71.14%. Comparing base (28930d4) to head (e026b62).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/mlj_serialization.jl	0.00%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   75.55%   71.14%   -4.42%     
==========================================
  Files           6        6              
  Lines         180      201      +21     
==========================================
+ Hits          136      143       +7     
- Misses         44       58      +14     

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[ablaom]

Looks good to me.