Fix WebDAVAdapter createDirectory (fix #1689)

[nicofrand]

This fixes several issue with directory creation:

1. flysystem-webdav - WebDAVAdapter.php - createDirectory() not working allways #1689 stopping on first "" (which is always the case when splitting the path beginning with a \)
2. When checking if the directory exists, the path prefixed with the DAV prefix does not contain the first \, resulting in the unprefixed path having its first char removed. Ex: "remote.php/webdav/user/directory" becomes "irectory".