Here is the corrected patch for caching the 301 redirections.

* It caches only the redirection not the response.
* It retains cacheable=True kwarg for http_error_301 method. ( I feel, it should be useful)
* Have made the cached dict as private.

I have updated the tests. The existing tests for 301 see no changes too.

If you have any review comments, please pitch in. (I shall add the docs and news entry before commit)

Thanks!