Path.join(["/", "", "foo"]) != Path.join(["/", "foo"])

This seems buggy:

``` elixir
iex(2)> ["/", "foo"] |> Path.join       
"/foo"
iex(3)> ["/", "", "foo"] |> Path.join
"foo"
```

Ruby:

``` ruby
irb(main):001:0> File.join("/", "", "foo")
=> "/foo"
```
