Workaround categorical merge

[kmax12]

This is a workaround for a possible bug in pandas where unordered categoricals that have a different order cannot merge correctly.

An issue has been filed in the pandas repo pandas-dev/pandas#22501

[codecov-io]

Codecov Report

Merging #231 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #231      +/-   ##
==========================================
+ Coverage   93.62%   93.62%   +<.01%     
==========================================
  Files          71       71              
  Lines        7654     7659       +5     
==========================================
+ Hits         7166     7171       +5     
  Misses        488      488

Impacted Files	Coverage Δ
...turetools/computational_backends/pandas_backend.py	92.25% <100%> (+0.14%)	⬆️

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@676b7cc). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #231   +/-   ##
=========================================
  Coverage          ?   93.63%           
=========================================
  Files             ?       71           
  Lines             ?     7661           
  Branches          ?        0           
=========================================
  Hits              ?     7173           
  Misses            ?      488           
  Partials          ?        0

Impacted Files	Coverage Δ
...turetools/computational_backends/pandas_backend.py	92.25% <100%> (ø)
featuretools/entityset/entity.py	89.77% <100%> (ø)
featuretools/entityset/entityset.py	93.59% <100%> (ø)

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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