test: enrich ut for pkg/protocol/header.go

[zstone12]

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提高对 hertz/pkg/protocol/header 的单测覆盖率

(Optional) More detail description for this PR(en: English/zh: Chinese).

en: hertz/pkg/protocol/header ut coverage increased from 55.73% to 80.4%
zh(optional): hertz/pkg/protocol/header 的单测覆盖率从 55.73% 提升到了 80.4%

Which issue(s) this PR fixes:

#257

[codecov]

Codecov Report

Base: 64.52% // Head: 66.17% // Increases project coverage by +1.64% 🎉

Coverage data is based on head (c1debde) compared to base (3a6bb4a).
Patch has no changes to coverable lines.

❗ Current head c1debde differs from pull request most recent head 31d1209. Consider uploading reports for the commit 31d1209 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #299      +/-   ##
===========================================
+ Coverage    64.52%   66.17%   +1.64%     
===========================================
  Files           85       85              
  Lines         8319     8319              
===========================================
+ Hits          5368     5505     +137     
+ Misses        2588     2455     -133     
+ Partials       363      359       -4     

Impacted Files	Coverage Δ
pkg/common/compress/compress.go	83.80% <0.00%> (+2.85%)	⬆️
pkg/protocol/header.go	73.85% <0.00%> (+17.51%)	⬆️

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[welkeyever]

lgtm