fix: Add charset to the "content-type" header

[Killian-fal]

📒 Description

Client.prepare_headers doesn't add the charset even if the config.encoding is feed.

Resolves #1230

🔗 What I've Done

1. Client.prepare_headers now add the config.encoding param if defined
2. Add a unit test

💬 Comments

X

🛫 Checklist

• Updated docs
• Added unit-tests
• Sample tests pass (run in local)
• W3C tests pass

[sonarqubecloud]

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 SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.97%. Comparing base (1fda878) to head (42b7f0e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1231   +/-   ##
=======================================
  Coverage   99.97%   99.97%           
=======================================
  Files         116      116           
  Lines        9393     9395    +2     
  Branches     1444     1445    +1     
=======================================
+ Hits         9391     9393    +2     
  Misses          1        1           
  Partials        1        1           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.