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
Also the documentation of $this->decodedChars should be updated then to mention its also used for query decoding.