fix cookie handling when following redirects

previous implementation was using the wrong key names for both the request and response namespaces.

I was surprised to find this bug because I guess it means no one was using this feature? Or perhaps this worked with a previous version of of Faraday which did some sort of conversion/normalization of the namespace further down the line? (looks like @mrmicahcooper, @mislav, and @evilmarty have made commits related to cookies — any insight into this?)

I only updated the existing tests. So, there's nothing stopping this new implementation from being wrong as well, other than that I tested it. This is theoretically fine because the namespace corresponds with the HTTP spec, which the previous version did not.

Let me know what you think.