One-hot encoder: fix bug with column names, so all data types are supported

[dsherry]

Fix #882

The bug was that sklearn's one-hot encoder's get_feature_names method requires the input to be str, but in the case when the input dataframe has a non-string index, we were trying to pass that index in, causing an error.

The fix is to first convert all the column names to str, then pass them into get_feature_names.

[codecov]

Codecov Report

Merging #897 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #897   +/-   ##
=======================================
  Coverage   99.78%   99.78%           
=======================================
  Files         195      195           
  Lines        8950     8966   +16     
=======================================
+ Hits         8931     8947   +16     
  Misses         19       19           

Impacted Files	Coverage Δ
...components/transformers/encoders/onehot_encoder.py	100.00% <100.00%> (ø)
...alml/tests/component_tests/test_one_hot_encoder.py	100.00% <100.00%> (ø)

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[angela97lin]

LGTM! Glad we got this in 😄