fix: avoid serializing body with buffer type

[pi0]

🔗 Linked issue

resolves #269

❓ 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

Normally we allow serializing pure objects (with object constructor) OR classes that implement toJSON().

However Buffer type is different and implements a toJSON method which is mainly for debugging.

This PR detects buffer type without depending on global Buffer as (Node.js) workaround

Note: Worth to mention that other BodyInit compatible types such as UInt8Array do not share this issue as do not implement toJSON. We might consider detecting them explicitly if any runtime in the feature does. (-- Investigating Readable Body stream in next steps, current check currently allows it to be unserialized --)

📝 Checklist

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

[codecov]

Codecov Report

Merging #273 (fe5157b) into main (b9ef395) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #273      +/-   ##
==========================================
+ Coverage   92.35%   92.39%   +0.04%     
==========================================
  Files           5        5              
  Lines         523      526       +3     
  Branches       97       99       +2     
==========================================
+ Hits          483      486       +3     
  Misses         40       40              

Files Changed	Coverage Δ
src/utils.ts	95.61% <100.00%> (+0.11%)	⬆️