Add Content-Encoding to metadata headers

[csw]

gofakes3 was not preserving the Content-Encoding header, meaning that clients would get raw gzipped data in GetObject responses instead of the expected decompressed data when it was stored with Content-Encoding: gzip. With this change, it matches the behavior of S3 itself.

[codecov-commenter]

Codecov Report

Merging #79 (fb90c9b) into master (f6a8a9a) will increase coverage by 0.03%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   66.67%   66.71%   +0.03%     
==========================================
  Files          28       28              
  Lines        2587     2590       +3     
==========================================
+ Hits         1725     1728       +3     
  Misses        610      610              
  Partials      252      252              

Impacted Files	Coverage Δ
cors.go	75.00% <ø> (ø)
gofakes3.go	64.03% <100.00%> (+0.18%)	:arrow_up:

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[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[johannesboyne]

Thanks @csw great PR!