title: Convert RDD[(K,V) to Map[K,List[V]]
body: <p>How can i convert a RDD of tuple2 (Key,Value) with duplicate Keys into a Map[K,List[V]] ?</p>
  
  <p>Input example:</p>
  
  <pre><code>val list = List((1,a),(1,b),(2,c),(2,d))
  val rdd = sparkContext.parallelize(list)
  </code></pre>
  
  <p>Output expected:</p>
  
  <pre><code>Map((1,List(a,b)),(2,List(c,d)))
  </code></pre>