Make headers flat.

[AYMENJD]

I see no point of exposing headers as a list of strings. HTTP already allows multiple header values to be represented as a single comma-separated value (RFC 9110), which is exactly how https://httpbin.org do.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.15%. Comparing base (5cc6f1e) to head (0e15654).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #236      +/-   ##
==========================================
+ Coverage   95.13%   95.15%   +0.01%     
==========================================
  Files          10       10              
  Lines        1831     1838       +7     
==========================================
+ Hits         1742     1749       +7     
  Misses         52       52              
  Partials       37       37              

Files with missing lines	Coverage Δ
httpbin/handlers.go	99.25% <100.00%> (ø)
httpbin/helpers.go	95.92% <100.00%> (+0.09%)	⬆️

🚀 New features to boost your workflow:

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