fix: avoid setting default content-type for responses with 304 status

[pi0]

• test: add failing test
• fix: avoid setting default content-type for responses with 304 status

🔗 Linked issue

resolves #603

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8345c1f) 77.80% compared to head (18c85ea) 77.82%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #641      +/-   ##
==========================================
+ Coverage   77.80%   77.82%   +0.02%     
==========================================
  Files          47       47              
  Lines        4281     4285       +4     
  Branches      610      611       +1     
==========================================
+ Hits         3331     3335       +4     
  Misses        933      933              
  Partials       17       17              

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