Fix HTTP Host header example

[AndreyVMarkelov]

Summary

• remove the URL scheme from the generated HTTP Host header
• add regression coverage for the rendered HTTP request

Why

HTTP Host header values contain only the authority hostname, not a URL scheme. The explorer currently generates Host: https://api.dropboxapi.com, which is invalid.

This supersedes #4 and preserves the fix originally proposed by @cakoose.

Validation

• npm run lint
• npm run typecheck
• npm test
• npm run build
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 14.26%. Comparing base (d43d850) to head (0456a44).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #336      +/-   ##
==========================================
+ Coverage   10.68%   14.26%   +3.58%     
==========================================
  Files           6        6              
  Lines        5775     5775              
  Branches        4       28      +24     
==========================================
+ Hits          617      824     +207     
+ Misses       5158     4950     -208     
- Partials        0        1       +1     

Flag	Coverage Δ
unit	14.26% <100.00%> (+3.58%)	⬆️

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