fix for check for content-type when gzip

[natrim]

this should work (it does for me)

closes #21

[codecov-io]

Codecov Report

Merging #22 into master will decrease coverage by 0.28%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
- Coverage   70.32%   70.04%   -0.29%     
==========================================
  Files           6        6              
  Lines         428      434       +6     
==========================================
+ Hits          301      304       +3     
- Misses        101      102       +1     
- Partials       26       28       +2

Impacted Files	Coverage Δ
lib/files.go	83.15% <55.55%> (-2.24%)	⬇️

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