fix(dev): don't overwrite proxy headers if already set

[MikeBellika]

🔗 Linked issue

#894

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 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)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Looks at X-Forwarded headers in proxyReq and doesn't overwrite them if they're present.

Resolves #894

📝 Checklist

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

[codecov]

Codecov Report

Merging #896 (b90aabf) into main (af0409f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #896      +/-   ##
==========================================
+ Coverage   67.49%   67.50%   +0.01%     
==========================================
  Files          59       59              
  Lines        5864     5866       +2     
  Branches      637      637              
==========================================
+ Hits         3958     3960       +2     
  Misses       1897     1897              
  Partials        9        9              

Impacted Files	Coverage Δ
src/dev/server.ts	75.20% <100.00%> (ø)
src/types/fetch.ts	100.00% <0.00%> (ø)

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[pi0]

Thanks!