[node-bridge] Fix multiple Set-Cookie headers in streaming mode

[TooTallNate]

Multiple Set-Cookie headers are being concat'd instead of being sent individually when in streaming mode.

Non-streaming:

$ curl -v https://01-remix-basics-eqztqyq3y.vercel-support.app/set-cookie 2>&1 | grep cookie
* h2h3 [:path: /set-cookie]
> GET /set-cookie HTTP/2
< set-cookie: foo=bar
< set-cookie: sessionId=38afes7a8

Streaming:

$ curl -v https://01-remix-basics-9b18alizh.vercel-support.app/set-cookie 2>&1 | grep cookie
* h2h3 [:path: /set-cookie]
> GET /set-cookie HTTP/2
< set-cookie: foo=bar,sessionId=38afes7a8

[styfle]

Thanks! It would be good to add a test 👍