Description:
------------
The method isOptional() doesn't return the correct boolean-value.

Reproduce code:
---------------
<?php

class Test {
  public function test( $a, $b = 1, $c ) {}
}

$reflection = new ReflectionClass( 'Test' );

$fparam = $reflection->getMethod( 'test' )->getParameters();

foreach( $fparam as $k => $v ) {
  echo $v->getName() . ' ';
  echo ( $v->isOptional() ? 'is optional' : 'is not optional' ) . "\n";
}

?>

Expected result:
----------------
a is not optional
b is optional
c is not optional


Actual result:
--------------
a is not optional
b is not optional
c is not optional