More robust handling when no data at a cutoff time

[kmax12]

This PR updates how we merge dataframes internally during feature computations.

Also added a new test case for the case this merge code now handles.

[codecov-io]

Codecov Report

Merging #234 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
+ Coverage   93.63%   93.63%   +<.01%     
==========================================
  Files          71       71              
  Lines        7661     7671      +10     
==========================================
+ Hits         7173     7183      +10     
  Misses        488      488

Impacted Files	Coverage Δ
...utational_backend/test_calculate_feature_matrix.py	99.28% <100%> (+0.01%)	⬆️
featuretools/entityset/entityset.py	93.6% <100%> (+0.01%)	⬆️

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[rwedge]

Looks good!