[2201.7.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 81.67%. Comparing base (46674b4) to head (c17463b).
Report is 1 commits behind head on 2201.7.x.

Additional details and impacted files

@@              Coverage Diff               @@
##             2201.7.x    #2182      +/-   ##
==============================================
+ Coverage       81.65%   81.67%   +0.02%     
  Complexity        577      577              
==============================================
  Files             393      393              
  Lines           21424    21430       +6     
  Branches         4803     4809       +6     
==============================================
+ Hits            17493    17504      +11     
+ Misses           2932     2928       -4     
+ Partials          999      998       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.