fix(h2): keep a request header named __proto__

[luantaraschi]

This relates to...

The h2 half of what I flagged in a comment on #5667. test/prototype-headers.js already covers this concern for client.request over HTTP/1.

Rationale

buildRequestHeaders in lib/dispatcher/client-h2.js probes the accumulator by index:

const current = headers[key]
...
headers[key] = current ? `${current}, ${val}` : val

For __proto__ that probe returns Object.prototype, which is truthy, so the header takes the "already present" branch and the value becomes "[object Object], pwned". That string is then assigned through the Object.prototype setter, which refuses a string, so nothing is stored and the header never reaches the wire.

Reproduced over a real HTTP/2 connection, before the change:

x-control arrived: sent
__proto__ arrived: no

__proto__ is a valid field name, and the flat array form is exactly what this function receives from the dispatcher.

There is no Object.prototype pollution: the setter refuses the string, so the global prototype is untouched.

Changes

The probe uses Object.hasOwn so an inherited name is not mistaken for an existing header, and all four writes go through a setHeader helper using Object.defineProperty, the guard parseHeaders already uses in lib/core/util.js.

Test added to test/prototype-headers.js, alongside the HTTP/1 cases: an h2 request carrying __proto__ reaches the server with that header, with a control header that already worked.

test/+(http2|h2)*.js 104 passing with 1 skipped as before, lint clean.

Features

N/A

Bug Fixes

An HTTP/2 request no longer drops a header named __proto__, and no longer corrupts its value with [object Object], first.

Breaking Changes and Deprecations

None.

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.45%. Comparing base (dd85997) to head (fdd13bd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5669      +/-   ##
==========================================
+ Coverage   93.43%   93.45%   +0.02%     
==========================================
  Files         110      110              
  Lines       38733    38795      +62     
==========================================
+ Hits        36190    36256      +66     
+ Misses       2543     2539       -4     

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