Update CacheHandler.php

This line looks suspicious. Shouldn't the `$request_var` start with a `/` ?
If I have `/cache/0/default/css/admin`, I'd rather have
  `$request_var = "/0/default/css/admin";`
than 
  `$request_var = "0/default/css/admin";`

I don't know how it works with Apache and others, but with Nginx, it simplifies things. Therefore:

  `if (!preg_match('#^/?([0-9]+)/([^/]+)/(.+)$#', $request_var, $matches)) {`

(drop the `?` if that is not needed)
Add test to allow leading slash in the cache URL regex
Merge pull request #5974 from hellekin/patch-1

Update CacheHandler.php