fix: set default value for accept

[imJYZ]

What is the purpose of the change

If there is no accept in the request header, set the default value.

refer to https://datatracker.ietf.org/doc/html/rfc2616#page-100

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.
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• 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.
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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

66.7% Coverage
0.0% Duplication

[codecov-commenter]

Codecov Report

Merging #13199 (801ce5e) into 3.2 (c4120fe) will increase coverage by 0.03%.
Report is 1 commits behind head on 3.2.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.2   #13199      +/-   ##
============================================
+ Coverage     69.40%   69.43%   +0.03%     
  Complexity        2        2              
============================================
  Files          1649     1649              
  Lines         68722    68725       +3     
  Branches      10012    10014       +2     
============================================
+ Hits          47696    47720      +24     
+ Misses        16454    16414      -40     
- Partials       4572     4591      +19     

see 43 files with indirect coverage changes

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