fix F.Option.toArray

* old Implementation __always throws ClassCastException__
* Java is not Scala. can not use Manifest. [java.util.Collection#toArray](http://bit.ly/ZGwS1m) return type SHOULD NOT override. can not create generic array in runtime.
* It should preserve the contracts of Collection#toArray. see javadoc http://bit.ly/11vSnzE