fix: compatibility with set headers option

[nuintun]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Breaking Changes

No breaking changes

[codecov]

Codecov Report

Merging #763 (860e57a) into master (441b449) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #763      +/-   ##
==========================================
+ Coverage   99.21%   99.23%   +0.01%     
==========================================
  Files          10       10              
  Lines         256      260       +4     
  Branches       82       83       +1     
==========================================
+ Hits          254      258       +4     
  Misses          2        2              

Impacted Files	Coverage Δ
src/middleware.js	100.00% <100.00%> (ø)

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[alexander-akait]

Thanks