client: support redirects in PropfindFlat()

One common method for CalDAV or CardDAV clients to find the current user
principal URL is to request the `/.well-known` URL (see [RFC 6764,
section 6][1]), expecting a redirect. Such URL is for example a valid
result of the discovery phase described in that RFC. The expectation is
that a client, given such URL, is able to find the principal URL by
following a redirect when sending a PROPFIND request.

This change makes `PropfindFlat()` (and, by extension,
`FindCurrentUserPrincipal()`) handle such a redirect and correctly
return the requested properties, even if their HREF is different from
the original request path.

[1]: https://datatracker.ietf.org/doc/html/rfc6764#section-6

internal/client.go

@@ -149,7 +149,11 @@ func (c *Client) PropfindFlat(path string, propfind *Propfind) (*Response, error
 		return nil, err
 	}
 
-	return ms.Get(c.ResolveHref(path).Path)
+	// If the client followed a redirect, the Href might be different from the request path
+	if len(ms.Responses) != 1 {
+		return nil, fmt.Errorf("PROPFIND with Depth: 0 returned %d responses", len(ms.Responses))
+	}
+	return &ms.Responses[0], nil
 }
 
 func parseCommaSeparatedSet(values []string, upper bool) map[string]bool {