Improve ArcanistUselessOverridingMethodXHPASTLinterRule by allowing overriding methods which set default values. For example, the following scenario is perfectly valid:

class SomeClass {
  public function __construct($x) {}
}

class SomeOtherClass extends Class {
  public function __construct($x = null) {
    parent::__construct($x);
  }
}

Added test case.