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 manually with urls that require redirects and cookies. This is theoretically fine because the namespace corresponds with the HTTP spec, which the previous version did not.

Let me know what you think.