In short: this removes all multiple slashes from your <B>PATH_INFOB>. Read on,
if you care why.
Multiple slashes in requests are a common problem, which many share.
Apparently, the RFC states that you should be able to expect different results
from http://server/ and http://server// (notice the second slash), so
if the frameworks wish to maintain RFC compatibility, they cannot remove those
extra slashes for you.
While you can handle this issue in a reverse proxy, in a rewrite module or
in your code, I find it more comfortable to have Plack take care of it in the
thin layer called Middlewares.
By enabling this middleware, all multiple slashes in your requests will
automatically be cut. //hello///world becomes /hello/world. Simple as