Verify canary release

• I verified that the issue exists in the latest Next.js canary release

Provide environment information

Not applicable, see reproduction here: https://codesandbox.io/s/condescending-torvalds-e3il3k?file=/package.json

What browser are you using? (if relevant)

No response

How are you deploying your application? (if relevant)

No response

Describe the Bug

Calling "cookies.set" twice on an API route with edge handler runtime will set only one cookie.

Expected Behavior

You'll see one "Set-Cookie" with 2 values, however it should be instead 2 "Set-Cookie" with only one value.

Set-Cookie can be repeated as many time as needed in an HTTP response to set multiple cookies. Edge middlewares do not suffer from this issue, it affects only API routes.

Link to reproduction

https://codesandbox.io/s/condescending-torvalds-e3il3k

To Reproduce

https://codesandbox.io/s/condescending-torvalds-e3il3k?file=/pages/index.tsx
Open index, check the Network tab for the "cookie" request.

_NEXT-471