serve webdav: fix --baseurl handling after lib/http refactor

The webdav library was confused by the Path manipulation done by
lib/http when stripping the prefix.

This patch adds the prefix back before calling it.

Fixes #6650

cmd/serve/webdav/webdav_test.go

@@ -51,6 +51,7 @@ func TestWebDav(t *testing.T) {
 	start := func(f fs.Fs) (configmap.Simple, func()) {
 		opt := DefaultOpt
 		opt.HTTP.ListenAddr = []string{testBindAddress}
+		opt.HTTP.BaseURL = "/prefix"
 		opt.Auth.BasicUser = testUser
 		opt.Auth.BasicPass = testPass
 		opt.Template.Path = testTemplate