Remove string casts

| Q             | A
| ------------- | ---
| Bug fix?      | partly (consistency, performance)
| New feature?  | no
| BC breaks?    | no because interface never said something else
| Deprecations? | |no
| Tests pass?   | yes
| Fixed tickets | #1182
| License       | MIT

Should wait for symfony/symfony#8927 to be merged. Although there is no real bc break, we might want to merge this for 2.0 only.