MockAgent unexpected multi header behaviour with fetch

[tobiasdcl]

First of all thank you for maintaining this library ❤️

Bug Description

I encountered an unexpected behaviour when using fetch with multi value headers via MockAgent.
I would expect the behaviour from fetch to be the same when using request (reference test) with MockAgent.
Is this behaviour expected and I am just missing something?

Reproducible By

save the following content as test/repro.js in the undici repository and then run npx tap test/repro.js

'use strict'

const { test } = require('tap')
const { setGlobalDispatcher, MockAgent } = require('..')
const { fetch } = require('..')

test("MockAgent - headers should be array of strings (fetch)", async (t) => {
  const mockAgent = new MockAgent();
  mockAgent.disableNetConnect();
  setGlobalDispatcher(mockAgent);

  const mockPool = mockAgent.get("http://localhost:3000");

  mockPool
    .intercept({
      path: "/foo",
      method: "GET",
    })
    .reply(200, "foo", {
      headers: {
        "set-cookie": ["foo=bar", "bar=baz", "baz=qux"],
      },
    });

  const { headers } = await fetch("http://localhost:3000/foo", {
    method: "GET",
  });

  t.equal(headers.getSetCookie(), ["foo=bar", "bar=baz", "baz=qux"]);
});

Expected Behavior

headers.getSetCookie() should return an array (["foo=bar", "bar=baz", "baz=qux"]) but instead it just returns ["foo=bar,bar=baz,baz=qux"] (array with one entry and all values concatenated)

Logs & Screenshots

 FAIL ​ test/repro.js
 ✖ should be equal

  test/repro.js                                                               
  27 |   });                                                                  
  28 |                                                                        
> 29 |   t.equal(headers.getSetCookie(), ["foo=bar", "bar=baz", "baz=qux"]);  
     | ----^                                                                  
  30 | });                                                                    

  --- expected                  
  +++ actual                    
  @@ -1,5 +1,3 @@               
   Array [                      
  -  "foo=bar",                 
  -  "bar=baz",                 
  -  "baz=qux",                 
  +  "foo=bar,bar=baz,baz=qux", 
   ]                            

  test: test/repro.js MockAgent - headers should be array of strings (fetch)

Environment

• Node v20.11.0
• ubuntu-22.04

Additional context

[metcoder95]

cc @KhafraDev

By doing this we can reproduce the issue:

const mockAgent = new MockAgent()
mockAgent.disableNetConnect()
setGlobalDispatcher(mockAgent)

const mockPool = mockAgent.get('http://localhost:3000')

mockPool
  .intercept({
    path: '/foo',
    method: 'GET'
  })
  .reply(200, 'foo', {
    headers: {
      'set-cookie': ['foo=bar', 'bar=baz', 'baz=qux']
    }
  })

fetch('http://localhost:3000/foo', {
  method: 'GET'
}).then(({ headers }) => {
  console.log(headers.getSetCookie())
})

Haven't dig into it yet.

[tobiasdcl]

Awesome, thank you! 🙏