Improve test coverage

[qnighy]

Why

What

[codecov]

Codecov Report

Merging #27 (b8724ce) into master (f602a37) will increase coverage by 5.07%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #27      +/-   ##
===========================================
+ Coverage   94.92%   100.00%   +5.07%     
===========================================
  Files           4         4              
  Lines         256       254       -2     
  Branches       48        46       -2     
===========================================
+ Hits          243       254      +11     
+ Misses          7         0       -7     
+ Partials        6         0       -6     

Impacted Files	Coverage Δ
lib/computed_model/model.rb	100.00% <100.00%> (+11.50%)	⬆️

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