OPDS: let user choose download path

This should fix #882
There is no need to check if a path is ended with a slash or not before joining its child node base name. According to the [UNIX specification](http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap03.html#tag_03_266) 
> Multiple successive slashes are considered to be the same as one slash.

So just add a "/" between the path name and the child node name. It should be no problem on Linux.