Redirect cache

When web clients like Firefox get a redirect they will not go after the old version any more even if you type it into the address bar.
This patch makes the requests framework work in the same way.
This helps performance when creating a request proxy that delegates to other servers based on URL.
This patch has me going from 245 requests per second to 470 requests per second against a server returning 308 response codes.

-- example server
https://gist.github.com/ericfrederich/a004862e1da1fb6916ef
-- example client
https://gist.github.com/ericfrederich/e1225e2d07e3ee923b27
-- example server output of 301 vs 302 redireects
https://gist.github.com/ericfrederich/b77bd4852a3cf9b968f0