pkg/headers: Move all explicit header keys into const

[Xuanwo]

Signed-off-by: Xuanwo github@xuanwo.io

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #236 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #236   +/-   ##
=======================================
  Coverage   84.29%   84.29%           
=======================================
  Files          12       12           
  Lines         917      917           
=======================================
  Hits          773      773           
  Misses         88       88           
  Partials       56       56           

Impacted Files	Coverage Δ
services/qingstor/servicer.go	77.03% <100.00%> (ø)

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