mkdirs() fails on some webdav servers

[andybelov]

A call to mkdirs('/some/folder/') will result in calling:

1. self.mkdir('/some')
2. self.mkdir('folder')

However some webdav (for example nginx) servers require a slash at the end of the path to folder:

1. self.mkdir('/some/')
2. self.mkdir('folder/')

RFC has no explicit requirement for a slash, but all the examples are given with it:
https://tools.ietf.org/html/rfc2518#page-33

So this is probably a good idea to add slashes in the end of path in mkdir calls.