fix: overriding headers in route handlers should work (#195)

* chore: add acceptance test

* chore: remove only from test

---------

Co-authored-by: Rob Stanford <me@robstanford.com>

Author: Skn0tt

tests/fixtures/simple-next-app/app/api/headers/route.js

@@ -0,0 +1,26 @@
+import { cookies } from 'next/headers'
+
+export const GET = async () => {
+  cookies().set('foo', 'foo1')
+  cookies().set('bar', 'bar1')
+
+  // Key, value, options
+  cookies().set('test1', 'value1', { secure: true })
+
+  // One object
+  cookies().set({
+    name: 'test2',
+    value: 'value2',
+    httpOnly: true,
+    path: '/handler',
+  })
+
+  // Cookies here will be merged with the ones above
+  return new Response('Hello, world!', {
+    headers: [
+      ['Content-Type', 'text/custom'],
+      ['Set-Cookie', 'bar=bar2'],
+      ['Set-Cookie', 'baz=baz2'],
+    ],
+  })
+}

tests/integration/simple-app.test.ts

@@ -94,3 +94,17 @@ test<FixtureTestContext>('handlers receive correct site domain', async (ctx) =>
   const url = new URL(data.url)
   expect(url.hostname).toBe('example.netlify')
 })
+
+// adapted from https://github.com/vercel/next.js/blob/bd605245aae4c8545bdd38a597b89ad78ca3d978/test/e2e/app-dir/actions/app-action.test.ts#L119-L127
+test<FixtureTestContext>('handlers can add cookies in route handlers with the correct overrides', async (ctx) => {
+  await createFixture('simple-next-app', ctx)
+  await runPlugin(ctx)
+  const index = await invokeFunction(ctx, { url: '/api/headers' })
+  expect(index.headers['content-type']).toEqual('text/custom')
+  const setCookieHeader = index.headers['set-cookie']
+  expect(setCookieHeader).toContain('bar=bar2; Path=/')
+  expect(setCookieHeader).toContain('baz=baz2; Path=/')
+  expect(setCookieHeader).toContain('foo=foo1; Path=/')
+  expect(setCookieHeader).toContain('test1=value1; Path=/; Secure')
+  expect(setCookieHeader).toContain('test2=value2; Path=/handler; HttpOnly')
+})