Fix slash removing in documentPathForRecursiveRef method

[bozaro]

Unfortunately path.Join remove last slash from result file.

For example:

path.Join("foo/bar/", "") == "foo/bar"

As result documentPathForRecursiveRef(current, "") returns rootDir without directory as file path and next relative path resolving lost last rootDir element.

In this workaround I try to restore last slash if needed.

[fenollp]

Please add regression tests.