fix: keep isTLS field in keepalive when reset the request

[welkeyever]

What type of PR is this?

fix

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• This PR title match the format: <type>(optional scope): <description>.
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo.

(Optional) Translate the PR title into Chinese.

长链接场景重置请求时保留，请求中和连接相关的字段（isTLS）

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

en:
There is a special filed (isTLS) in the request, which is a link-related field, and its life cycle follows the link. Therefore, in the keepalive scenario, this field needs to be skipped when resetting the request, otherwise it will affect the scheme judgment of the subsequent request of the long link.

zh(optional):
请求里有个特殊filed（isTLS），它是一个链接相关字段，生命周期跟随链接，因此在长链接复用场景，重置请求的时候需要跳过该字段，否则影响长链接后续请求的scheme判断。

(Optional) Which issue(s) this PR fixes:

fixes #874

(Optional) The PR that updates user documentation:

[codecov]

Codecov Report

Patch coverage: 44.44% and project coverage change: +0.13% 🎉

Comparison is base (3161f04) 79.74% compared to head (c603e78) 79.87%.

❗ Current head c603e78 differs from pull request most recent head d52fb68. Consider uploading reports for the commit d52fb68 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #876      +/-   ##
===========================================
+ Coverage    79.74%   79.87%   +0.13%     
===========================================
  Files           97       97              
  Lines         9534     9541       +7     
===========================================
+ Hits          7603     7621      +18     
+ Misses        1432     1423       -9     
+ Partials       499      497       -2     

Files Changed	Coverage Δ
pkg/protocol/request.go	82.86% <37.50%> (-0.99%)	⬇️
pkg/app/context.go	85.48% <100.00%> (ø)

... and 3 files with indirect coverage changes

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