neo4j: Sanitize the inputs

![](http://imgs.xkcd.com/comics/exploits_of_a_mom.png )
— xkcd

Signed-off-by: Jean-Christophe Beaupré <jcbrinfo@users.noreply.github.com>
There is 2 tests: one with a slash (kept intact), and another without. The idea behind this edition is that previous versions crashed with the slash. The commit address this problem by using `join_path`.