Pathname#cleanpath is a purely lexical cleanup of a unix path, to remove
useless syntax and return an equivalent path. However, the current default
algorithm makes choices that can return semantically different paths.

This patch sets the default algorithm to the safe alternative, guaranteeing the
return of a semantically identical path. The unsafe/aggressive algorithm
remains and can be enabled where appropriate.

For added context, here's an example of how this can currently fail: https://gist.github.com/4069375

Closing this, please refer to Feature #7401 in redmine