fix(proxy-agent): preserve repeated iterable headers

[GiHoon1123]

Follow-up to #5758.

ProxyAgent converts iterable request headers to an object before checking for Proxy-Authorization. If the iterable contains the same header more than once, the conversion keeps only the last value.

Keep repeated values when building the headers used by the proxy request, and add a regression test for the case.

Tests:

• npx borp --timeout 180000 test/proxy-agent.js
• npx eslint lib/dispatcher/proxy-agent.js test/proxy-agent.js

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.51%. Comparing base (b73952a) to head (b7026b5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5779      +/-   ##
==========================================
- Coverage   93.51%   93.51%   -0.01%     
==========================================
  Files         110      110              
  Lines       39359    39372      +13     
==========================================
+ Hits        36808    36820      +12     
- Misses       2551     2552       +1     

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