Trend fix

[gsheni]

With the new custom primitives, any aggregation with two input types, needs to updated to reflect the new changes. The Trend primitive was one that was forgotten to be fixed.
Furthermore, fixed an aggregation primitive test.

[codecov-io]

Codecov Report

Merging #22 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   86.83%   86.86%   +0.03%     
==========================================
  Files          73       73              
  Lines        6768     6778      +10     
==========================================
+ Hits         5877     5888      +11     
+ Misses        891      890       -1

Impacted Files	Coverage Δ
...ols/tests/feature_function_tests/test_agg_feats.py	98.35% <100%> (ø)	⬆️
featuretools/primitives/aggregation_primitives.py	92.1% <100%> (ø)	⬆️
featuretools/entityset/serialization.py	17.64% <0%> (-1.2%)	⬇️
featuretools/tests/testing_utils/mock_ds.py	94.54% <0%> (ø)	⬆️
...sts/feature_function_tests/test_direct_features.py	100% <0%> (ø)	⬆️
featuretools/synthesis/encode_features.py	94.73% <0%> (ø)	⬆️
featuretools/demo/__init__.py	100% <0%> (ø)	⬆️
featuretools/tests/testing_utils/__init__.py	100% <0%> (ø)	⬆️
featuretools/demo/api.py	100% <0%> (ø)	⬆️
...tests/computational_backend/test_pandas_backend.py	89.38% <0%> (ø)	⬆️
... and 62 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fb87937...0dd8acf. Read the comment docs.