Fix handling weights in supervised fit data scitype

[ablaom]

This PR fixes a flaw in the definition of supervised_fit_data_scitype which defines the fallback fit_data_scitype for Supervised models. The problem concerned the case supports_weights(model)=true or supports_class_weights(model)=true.

This PR supports JuliaAI/MLJBase.jl#731

cc @pazzo83

[codecov-commenter]

Codecov Report

Merging #137 (c2b4a66) into dev (2a77b27) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev     #137   +/-   ##
=======================================
  Coverage   87.29%   87.29%           
=======================================
  Files           8        8           
  Lines         299      299           
=======================================
  Hits          261      261           
  Misses         38       38           

Impacted Files	Coverage Δ
src/model_traits.jl	71.69% <0.00%> (ø)

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