rack plugin: rack3 header values can be an Array to handle multiple values

[niol]

There is one changed feature in Rack 3 which is not backwards compatible:

• Response header values can be an Array to handle multiple values (and no longer supports \n encoded headers).

This patch fixes seing in response header:

Set-Cookie: ["autologin=[secret];secure"]

instead of

Set-Cookie: autologin=[secret];secure