Add a new test to fix issue #628

[humphd]

This change adds a test to fix the case that the data we send to writeFile is a Number vs. a String of Buffer.

[codecov-io]

Codecov Report

Merging #673 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   86.71%   86.76%   +0.05%     
==========================================
  Files          16       16              
  Lines        1746     1746              
==========================================
+ Hits         1514     1515       +1     
+ Misses        232      231       -1

Impacted Files	Coverage Δ
src/filesystem/implementation.js	83.81% <0%> (+0.09%)	⬆️

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