title: Why Map withDefaultValue returns Option = None
body: <p>I'm using Scala 2.10.4 and I cannot figure out why I cannot get the default value for non existent keys. What I get instead is <code>Option[Int]</code> with value of <code>None</code>.</p>
  
  <pre><code>  val m = Map[String, Int]().withDefaultValue(0) 
  //&gt; m  : scala.collection.immutable.Map[String,Int] = Map()
    m.get("ss") 
  //&gt; res0: Option[Int] = None
  </code></pre>
  
  <p>I expected res0 to be <code>0</code>. </p>
  
  <p>Why it is not?
  How can I obtain the expected behaviour?</p>