Use floor() instead of round() in %%

[UchidaMizuki]

Fix #364.
I have enhanced tests for %% to make sure it behaves correctly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f56521c) 91.54% compared to head (6ee538e) 91.58%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
+ Coverage   91.54%   91.58%   +0.03%     
==========================================
  Files          19       19              
  Lines         994      998       +4     
==========================================
+ Hits          910      914       +4     
  Misses         84       84              

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[Enchufa2]

Good catch, thanks!