BUG: fix the initialize method for KerasClassifier

[adriangb]

Overrides initialize in KerasClassifier to properly handle the classes param of partial_fit.

[codecov-io]

Codecov Report

Merging #140 (649ac13) into master (94960da) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #140      +/-   ##
==========================================
+ Coverage   99.33%   99.50%   +0.17%     
==========================================
  Files           5        5              
  Lines         599      609      +10     
==========================================
+ Hits          595      606      +11     
+ Misses          4        3       -1     

Impacted Files	Coverage Δ
scikeras/utils/transformers.py	100.00% <100.00%> (ø)
scikeras/wrappers.py	99.15% <100.00%> (+0.30%)	⬆️

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