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
I don't agree that decoding `?` makes things better. For example: `/path/sub?foo?bar` it gets very hard to see where the query part starts. I think other chars are more worth to keep decoded like `@`, e.g. when you pass an email like `?to=me@example.org`. So I think its ok to use `$this->decodedChars` BUT without `=` and `&` (in case somebody added it in a subclass). Just unset them in a local variable.