Add support for stale directive for digest authenticator

[hustern]

Hi studio-b12 members !

First of all, I'd like to thank you for this wonderful/easy-to-use Webdav Go package 😃

I'm opening this PR because I recently faced an issue trying to connect to a Webdav server using Digest auth mechanism.
Client successfully connected to this server using digest authenticator, but after some time it was rejected by the server with 401 status codes.

I took a look at the RFC2617 (Section 3.2.1 The WWW-Authenticate Response Header), and it seems that the server can invalidate the generated nonce value after some time (which is my case). However, in such case, the stale directive is added to the WWW-Authenticate header, indicating to the client to re-authenticate without reprompting the user for username and password.

So I forked your project and did a small change in the Verify method of the DigestAuth authenticator to handle such case. I thought you might be interested with these changes 😉

[chripo]

You're welcome.
Tanks for the improvement and your Tests.
LGTM.