Optimize parseCharset method for best performance.

[RainYuY]

What is the purpose of the change?

Optimize parseCharset method for best performance.
This is add optimize for pr #15188

Checklist

• Make sure there is a GitHub_issue field for the change.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test to verify your logic correction. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Make sure gitHub actions can pass. Why the workflow is failing and how to fix it?

[RainYuY]

PTAL @oxsean

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.78%. Comparing base (317bee1) to head (44e6952).
⚠️ Report is 202 commits behind head on 3.3.

Files with missing lines	Patch %	Lines
...o/remoting/http12/message/DefaultHttpResponse.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.3   #15203      +/-   ##
============================================
+ Coverage     60.75%   60.78%   +0.03%     
- Complexity    10885    10901      +16     
============================================
  Files          1885     1885              
  Lines         86071    86074       +3     
  Branches      12892    12893       +1     
============================================
+ Hits          52292    52324      +32     
+ Misses        28325    28299      -26     
+ Partials       5454     5451       -3     

Flag	Coverage Δ
integration-tests	33.17% <0.00%> (+0.03%)	⬆️
samples-tests	29.20% <0.00%> (-0.02%)	⬇️
unit-tests	58.92% <83.33%> (+0.02%)	⬆️

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