chore(volo-http): add more unit tests for server

[yukiiiteru]

Motivation

Add more unit tests

[codecov]

Codecov Report

❌ Patch coverage is 98.12500% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.94%. Comparing base (663fd5c) to head (1880acf).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
volo-http/src/server/test_helpers.rs	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
+ Coverage   40.85%   41.94%   +1.08%     
==========================================
  Files         160      160              
  Lines       19646    19806     +160     
==========================================
+ Hits         8026     8307     +281     
+ Misses      11620    11499     -121     

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