Do not cache results for non-string names

[sokra]

fixes #99

[webpack-bot]

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[codecov]

Codecov Report

Merging #100 into master will increase coverage by 0.09%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   94.29%   94.38%   +0.09%     
==========================================
  Files          37       37              
  Lines        1421     1426       +5     
  Branches      310      312       +2     
==========================================
+ Hits         1340     1346       +6     
+ Misses         81       80       -1

Impacted Files	Coverage Δ
lib/CachedInputFileSystem.js	93.98% <80%> (+0.73%)	⬆️

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