[PropertyAccessor] Wrong number of parameters

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Since [6d2af217aafd03e3f1600ce0ebc9c30cf0a7fc70#diff-3e45cf556c0330623c0167841d303dff](https://github.com/symfony/symfony/commit/6d2af217aafd03e3f1600ce0ebc9c30cf0a7fc70#diff-3e45cf556c0330623c0167841d303dff) the PropertyAccessor is failing on setters with default values because  [ReflectionFunctionAbstract::getNumberOfRequiredParameters](http://www.php.net/manual/es/reflectionfunctionabstract.getnumberofparameters.php) only count the parameters without default values.