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

[zstone12]

What type of PR is this?

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• [x] This PR title match the format: <type>(optional scope): <description>
• [x] The description of this PR title is user-oriented and clear enough for others to understand.

(Optional) Translate the PR title into Chinese.

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:

https://github.com/cloudwego/hertz/issues/257