Option Arbitrary delegates to Option Gen

Resolves typelevel#401.

src/main/scala/org/scalacheck/Arbitrary.scala

@@ -68,7 +68,7 @@ object Arbitrary extends ArbitraryLowPriority with ArbitraryArities {
 
 /** separate trait to have same priority as ArbitraryArities */
 private[scalacheck] sealed trait ArbitraryLowPriority {
-  import Gen.{const, choose, sized, frequency, oneOf, buildableOf, resize}
+  import Gen.{const, choose, sized, frequency, oneOf, buildableOf}
 
   /** Creates an Arbitrary instance */
   def apply[T](g: => Gen[T]): Arbitrary[T] = new Arbitrary[T] {
@@ -336,13 +336,7 @@ private[scalacheck] sealed trait ArbitraryLowPriority {
 
   /** Arbitrary instance of the Option type */
   implicit def arbOption[T](implicit a: Arbitrary[T]): Arbitrary[Option[T]] =
-    Arbitrary(sized(n =>
-      // When n is larger, make it less likely that we generate None,
-      // but still do it some of the time. When n is zero, we always
-      // generate None, since it's the smallest value.
-      frequency(
-        (n, resize(n / 2, arbitrary[T]).map(Some(_))),
-        (1, const(None)))))
+    Arbitrary(Gen.option(a.arbitrary))
 
   /** Arbitrary instance of the Either type */
   implicit def arbEither[T, U](implicit at: Arbitrary[T], au: Arbitrary[U]): Arbitrary[Either[T, U]] =