Feature Request: Support for out-of-spec unquoted getetag values

[EtienneBruines]

Some WebDAV servers do not return quoted etag values (to make them re-usable for the ETag HTTP response header), e.g. haiwen/seafdav#82 and photoprism/photoprism#3495 even though they should.

Would it be possible/reasonable to accept unquoted ETag values? To make this library compatible with a wider range of servers?

Potentially like #69 although I was thinking modifying

go-webdav/internal/elements.go

Line 382 in 810c51f

return fmt.Errorf("webdav: failed to unquote ETag: %v", err)

to not return an error if it's simply an alphanumerical string (a hash or something) that just doesn't have quotes.

What do you think?