use modern redirect status

[davidism]

307 (temporary) and 308 (permanent) status codes preserve the method and body of the original request. #1342 updated the routing system to use 308 instead of 301 for slash redirects. redirect should be updated from 302 to 307, and append_slash_redirect should be updated from 301 to 308.

[ThiefMaster]

Just to clarify: this is just for internal routing-related redirects, so e.g. flask's redirect() won't be affected?

Because changing "normal" redirects to suddenly preserve a POST method would likely be a problem - redirect-after-post is super common when submitting forms...

[davidism]

Good point, I won't update redirect, only append_slash_redirect. Routing strict_slashes were already updated in #1342.