Revert "normalize root path for use on all OS's"

This reverts commit f05c05f10635f0c2d313b6807f58bb044c47d1ca.

When looking into koa-send, I found out the source of this problem was already fixed by Jonathan Ong 4 days ago (see koajs/send@7ba7cc3) so my commit here should be reverted, because it's superfluous.