Webdav server in a prefixed url

[Ph0tonic]

When the root of a webdav server is hosted like https://example.com/sub/root/, and we query the root folder, the name of the resource in a PROPFIND response is / and not /sub/root/ leading to a key error.

I would propose to add an optional parameter in the constructor, would it be ok ? Or should we prefer another way ? I can make a PR.

[pataquets]

FYI: #171 might be somewhat related