Added failing test for ParameterBag#getInt

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets |
| License       | MIT
| Doc PR        | 

`ParameterBag::getInt` will cast the default value to an integer which is problematic when you want `null` to be returned when the key is not present. https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/ParameterBag.php#L253