URL-encoded slashes don't work for vault IDs or document IDs

[DRK3]

If your vault ID or document ID contains a slash, the resulting location that will be returned from the server will be unreachable since the slash breaks the path. Even if the slash is urlencoded as %2F, it still can't be reached.

This seems to be an inherent issue in the Go net/url and so this issue carries over to the Gorilla mux package. See gorilla/mux#77 for details.

There is a workaround: call mux.useEncodedPath() before starting the router.

We will also probably want to urlencode the returned location in the response so that it's actually usable as well.