Fix XMLHttpRequest.setRequestHeader to append duplicate headers per spec

packages/react-native/Libraries/Network/XMLHttpRequest.js

@@ -509,7 +509,9 @@ class XMLHttpRequest extends EventTarget {
     if (this.readyState !== this.OPENED) {
       throw new Error('Request has not been opened');
     }
-    this._headers[header.toLowerCase()] = String(value);
+    const key = header.toLowerCase();
+    const existing = this._headers[key];
+    this._headers[key] = existing ? existing + ', ' + String(value) : String(value);
   }
 
   /**

packages/react-native/Libraries/Network/__tests__/XMLHttpRequest-test.js

@@ -294,6 +294,32 @@ describe('XMLHttpRequest', function () {
     expect(GlobalPerformanceLogger.stopTimespan).not.toHaveBeenCalled();
   });
 
+  it('should append values when setRequestHeader is called with the same header', function () {
+    xhr.open('GET', 'blabla');
+    xhr.setRequestHeader('X-Custom', 'value1');
+    xhr.setRequestHeader('X-Custom', 'value2');
+
+    // $FlowFixMe[prop-missing]
+    expect(xhr._headers['x-custom']).toBe('value1, value2');
+  });
+
+  it('should lowercase header names in setRequestHeader', function () {
+    xhr.open('GET', 'blabla');
+    xhr.setRequestHeader('Content-Type', 'application/json');
+    xhr.setRequestHeader('content-type', 'text/plain');
+
+    // $FlowFixMe[prop-missing]
+    expect(xhr._headers['content-type']).toBe(
+      'application/json, text/plain',
+    );
+  });
+
+  it('should throw when setRequestHeader is called before open', function () {
+    expect(() => {
+      xhr.setRequestHeader('foo', 'bar');
+    }).toThrow('Request has not been opened');
+  });
+
   it('should sort and lowercase response headers', function () {
     // Derived from XHR Web Platform Test: https://github.com/web-platform-tests/wpt/blob/master/xhr/getallresponseheaders.htm
     xhr.open('GET', 'blabla');