No support for Bearer Token authentication

[peppermenta]

Hello Collaborators,

Describe the bug
401 Authorization error on trying to connect to a WebDAV server with Bearer type of authentication

Software

• OS: Ubuntu 20.04 LTS
• Golang: go version go1.19.2 linux/amd64
• Version: latest studio-b12/gowebdav (loaded via go get)

To Reproduce

1. Connect to any WebDAV server which requires Bearer type of authentication. I used OwnCloud
2. Try to read the list of directories using c.ReadDir("/") (Any other operation also fails)
3. Error: 401 Authorization Error

Expected
No authorization error, the client should be able to connect to the server and execute any operation

Additional context
This seems to be due the fact that only two types of auth: Basic and Digest are supported.
I would be willing to work on a PR to add Bearer authentication support

[chripo]

Sounds good to me.
This should be straightforward.
PR welcome.

[chripo]

Should be much easier now.