Fix / in smart-variable param representation

The slash in the to_param representation was misleading ActionPack
to believe into an additional REST nesting level.

Three possible fixes to that problem:
1) Patch ActionPack to url escape, if appropriate
2) Replace the slash by a neutral character
3) Escape the slash, into %2F.
Actually escape twice, into %252F...

I chose the second solution, using a backslash.