Fix root path handling for WebDAV ext storage

Added missing cleanPath() call that converts "/" to "" when calling
SabreDAV. This is needed because SabreDAV will discard its base URL when
passing "/".

Please review @DeepDiver1975 @icewind1991 @MorrisJobke 

Note: if you run the unit tests locally there's only one unrelated failing test related to mimetypes.
But this PR fixes all the other failing ones.

To test:
1) Setup another OC instance
2) Edit "apps/files_external/tests/config.php" and put the config in webdav and owncloud
3) Run unit test for "apps/files_external/tests/webdav.php" and "apps/files_external/tests/owncloud.php"