Fix XMLHttpRequest.setRequestHeader to append duplicate headers per spec

[zmunm]

Summary:

XMLHttpRequest.setRequestHeader() overwrites the previous value when called multiple times with the same header name. Per the XHR Living Standard §4.5.2, duplicate headers should be appended with , .

Real-world example: Sentry JS SDK assumes append behavior
when setting the baggage header, which causes previously set baggage values to be silently dropped on React Native.

Changelog:

[GENERAL] [FIXED] - Fix XMLHttpRequest.setRequestHeader to append duplicate headers per spec

Note: This is technically a breaking change for code that relied on the previous overwrite behavior (e.g., calling setRequestHeader twice to replace a value). However, that behavior was non-compliant with the XHR spec, so such code should use a single call with the desired final value instead.

Test Plan:

Unit tests — 3 tests added in XMLHttpRequest-test.js:

• Appends values when same header is set twice
• Merges case-insensitively
• Throws when called before open()

Run the snippet below in both browser DevTools and RN debugger console to compare:

const xhr = new XMLHttpRequest();
xhr.open('GET', 'https://httpbin.org/headers');
xhr.setRequestHeader('X-Test', 'value1');
xhr.setRequestHeader('X-Test', 'value2');
xhr.onload = () => {
  const headers = JSON.parse(xhr.responseText).headers;
  console.log('X-Test:', headers['X-Test']);
  // Before: "value2"
  // After:  "value1, value2"
};
xhr.send();

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[meta-codesync]

@fabriziocucci has imported this pull request. If you are a Meta employee, you can view this in D100144689.

[fabriziocucci]

This landed on main in 58bf844 already.

Thanks @zmunm for the fix! 🙌