[2201.9.x] Fix Content-Type header duplication via addHeader method

[TharmiganK]

Purpose

Part of: ballerina-platform/ballerina-library#7268

Examples

N/A

Checklist

• Linked to an issue
• Updated the changelog
• Added tests
• Updated the spec
• Checked native-image compatibility
• Checked the impact on OpenAPI generation

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.90%. Comparing base (d852de2) to head (a02a344).
Report is 7 commits behind head on 2201.9.x.

Additional details and impacted files

@@              Coverage Diff               @@
##             2201.9.x    #2183      +/-   ##
==============================================
- Coverage       80.93%   80.90%   -0.03%     
  Complexity        561      561              
==============================================
  Files             391      391              
  Lines           21641    21647       +6     
  Branches         4988     4994       +6     
==============================================
- Hits            17515    17514       -1     
- Misses           3113     3122       +9     
+ Partials         1013     1011       -2     

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