Support for Rack 3 headers formatted as arrays

[Martin Posthumus]

Hello,

As of Rack v3, the internal representation of headers containing 
multiple values appears to have been changed to use an array of strings 
rather than a newline-separated string. Pull request visible here: 
https://github.com/rack/rack/pull/1793

The Rack changelog (https://github.com/rack/rack/blob/main/CHANGELOG.md) 
also includes this entry under 3.0.0:
"Response header values can be an Array to handle multiple values (and 
no longer supports \n encoded headers)."



I'm running into some serialization issues with this sort of header when 
trying to run an application on Unicorn that makes use of multiple 
cookies. The `set-cookie` header is returning what appears to be a 
stringified array:

set-cookie: ["my.cookie=.....; domain=......; path=/", 
"rack.session=......; path=/; httponly"]

Which in turn results in cookies getting registered with names like 
`["rack.session` rather than `rack.session`.


I'm not especially familiar with Unicorn's internals, but poking around 
a little bit, it looks like this might be related to the definition of 
`http_response_write` in lib/unicorn/http_response.rb, where it handles 
newline-separated strings, but not arrays. I can confirm that the 
set-cookie header value appears to be an array in this method, not a string.


At present I'm only seeing this issue when running on a Unicorn server. 
When I swap out something like webrick, it appears the array values are 
handled as expected.


Thank you,

Martin