## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
Fixes#30430
There's some more discussion in the issue, but in summary:
- web `Headers` implementation combines all header values with `', '`
- For `Set-Cookie` headers, you're supposed to set them as separate values, not combine them
- web `Headers` forbids the use of `Cookie`, `Set-Cookie` and some more headers, so they don't have custom implementation for those, and still joins them with `,`
- We currently just split them using `split(',')`, but this breaks when the header contains a date (expires, max-age) that also includes a `,`
I used this method to split the Set-Cookie header properly: https://www.npmjs.com/package/set-cookie-parser#splitcookiestringcombinedsetcookieheader as suggested [here](https://github.com/whatwg/fetch/issues/973#issuecomment-559678813)
I didn't add it as a dependency, since we only needed that one method and I wasn't sure what the process is for adding dependencies, so I just added the method in the middleware utils
