fix(mock-agent): split set-cookie (nodejs#2619)

lib/mock/mock-utils.js

@@ -188,11 +188,21 @@ function buildKey (opts) {
 }
 
 function generateKeyValues (data) {
-  return Object.entries(data).reduce((keyValuePairs, [key, value]) => [
-    ...keyValuePairs,
-    Buffer.from(`${key}`),
-    Array.isArray(value) ? value.map(x => Buffer.from(`${x}`)) : Buffer.from(`${value}`)
-  ], [])
+  const keys = Object.keys(data)
+  const result = []
+  for (let i = 0; i < keys.length; ++i) {
+    const key = keys[i]
+    const value = data[key]
+    const name = Buffer.from(`${key}`)
+    if (Array.isArray(value)) {
+      for (let j = 0; j < value.length; ++j) {
+        result.push(name, Buffer.from(`${value[j]}`))
+      }
+    } else {
+      result.push(name, Buffer.from(`${value}`))
+    }
+  }
+  return result
 }
 
 /**

test/mock-agent.js

@@ -2624,3 +2624,33 @@ test('MockAgent - Sending ReadableStream body', async (t) => {
 
   t.same(await response.text(), 'test')
 })
+
+// https://github.com/nodejs/undici/issues/2616
+test('MockAgent - headers should be array of strings (fetch)', async (t) => {
+  t.plan(1)
+
+  const mockAgent = new MockAgent()
+  mockAgent.disableNetConnect()
+  setGlobalDispatcher(mockAgent)
+
+  t.teardown(mockAgent.close.bind(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 response = await fetch('http://localhost:3000/foo', {
+    method: 'GET'
+  })
+
+  t.same(response.headers.getSetCookie(), ['foo=bar', 'bar=baz', 'baz=qux'])
+})