Middleware should merge API route response cookies

[ascorbic]

Currently, calling res.cookies.set in middleware replaces any cookies set by an API route. Instead they should be merged.

e2e test: skip-trailing-slash-redirect

[taty2010]

Test:
https://github.com/netlify/next-runtime/blob/e02c2b7e5555805d499b541feda2dbbf94da4a4f/test/e2e/modified-tests/skip-trailing-slash-redirect/index.test.ts

App:
https://github.com/netlify/next-runtime/blob/e02c2b7e5555805d499b541feda2dbbf94da4a4f/test/e2e/modified-tests/skip-trailing-slash-redirect/app/pages/api/test-cookie.js

https://github.com/netlify/next-runtime/blob/e02c2b7e5555805d499b541feda2dbbf94da4a4f/test/e2e/modified-tests/skip-trailing-slash-redirect/app/middleware.js

[ascorbic]

The specific test is this one, which you can enable by setting RUN_SKIPPED_TESTS