GET Parameters not getting Cached on Multisite Subsites where the whole multisite has a directory path.

## Setup

* Wordpress Multisite
* The Main blog can be reached via `mydomain.com/blog`
* The Subsite can be reached via `mydomain.com/blog/at`

## Behaviour

Pages with GET Parameters don't get cached for the subsites.
i.e.: `mydomain.com/blog/at?test` will not create a cache file

## Suspected Cause

In wp-cache-base.php:22:
https://github.com/Automattic/wp-super-cache/blob/32ec89c62f29141987e5465d8bb117d2c6b5fe4d/wp-cache-base.php#L22

In my case the `$current_blog->path` will return `/blog/at/` and the `/` will not get removed.
This value is later used for the file name of the cache file. Since a filename cannot contain a `/` the creation of the cache file fails.