fix: lowerCase header

[imJYZ]

What is the purpose of the change

Compatible with lowercase request headers

Brief changelog

Verifying this change

Checklist

• Make sure there is a GitHub_issue field for the change (usually before you start working on it). Trivial changes like typos do not require a GitHub issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Each commit in the pull request should have a meaningful subject line and body.
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• Check if is necessary to patch to Dubbo 3 if you are work on Dubbo 2.7
• Write necessary unit-test to verify your logic correction, more mock a little better when cross module dependency exist. If the new feature or significant change is committed, please remember to add sample in dubbo samples project.
• Add some description to dubbo-website project if you are requesting to add a feature.
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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

83.3% Coverage
0.0% Duplication

[codecov-commenter]

Codecov Report

Merging #13247 (3683007) into 3.2 (ad26a69) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.2   #13247      +/-   ##
============================================
+ Coverage     69.49%   69.51%   +0.02%     
  Complexity        2        2              
============================================
  Files          1653     1653              
  Lines         68937    68939       +2     
  Branches      10045    10046       +1     
============================================
+ Hits          47907    47926      +19     
+ Misses        16398    16387      -11     
+ Partials       4632     4626       -6     

see 34 files with indirect coverage changes

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