fix: allow multiple set-cookie headers to be set before render

[deer]

closes #2346

The primary issue was the snippet headers[key] = value. This caused only one set-cookie to make it through. Switching to a Headers object allowed me to use append when appropriate, allowing for multiple set-cookie entries.

I also cleaned up one of the tests to make use of steps, because I originally broke it and couldn't immediately tell which part was failing.

[marvinhagemeister]

Nice catch!