Add vary header in case of gzip or brotly enabled

[istarkov]

Closes #94

[lukeed]

Thank you~ Surprised this hasn't come up sooner

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.52%. Comparing base (cf787b9) to head (d2069df).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   98.52%   98.52%           
=======================================
  Files           2        2           
  Lines          68       68           
=======================================
  Hits           67       67           
  Misses          1        1           

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