fix: resp set trailer will panic

[wzekin]

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fix

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修复当使用 ResponseHeader.Set/Add 设置 Trailer 时，有可能会 panic 的问题

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

en: ResponseHeader.Set/Add method uses bytesconv.S2b method to reduce string copying, while SetTrailer modifies data in place using utils.NormalizeHeaderKey. However, if the string used for setting the header comes from RODATA, it can cause a segmentation fault.
Solution: make a copy of the header value when calling SetTrailer, to avoid modifying the data in place.

zh(optional): ResponseHeader.Set/Add 使用了 bytesconv.S2b 方法来减少 string 的拷贝，而 SetTrailer 会使用 utils.NormalizeHeaderKey 来原地修改数据，这是如果设置 Header 使用的 string 来自 RODATA，那么会造成段错误
解决方案：在 SetTrailer 时拷贝一次 header value

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.81%. Comparing base (b7cbc9d) to head (7a9b04a).
Report is 13 commits behind head on develop.

❗ Current head 7a9b04a differs from pull request most recent head c41a858. Consider uploading reports for the commit c41a858 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1102      +/-   ##
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+ Coverage    82.79%   82.81%   +0.02%     
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  Files           98       98              
  Lines        10032    10033       +1     
===========================================
+ Hits          8306     8309       +3     
+ Misses        1236     1235       -1     
+ Partials       490      489       -1     

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