decodes some special chars in a URL query

| Q             | A
| ------------- | ---
| Bug fix?      | yes|no (not sure :) )
| New feature?  | no
| BC breaks?    | yes|no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12200
| License       | MIT
Damn `+` is probably risky as well. Since it is the encoded form of space when using forms.
So `?foo=bar+baz` will result in `_GET["foo"] = 'bar baz'` which is bad.

So I guess we should just decode the `/` for now.