title: Behaviour of withDefaultValue in mutable.Map
body: <p>Can anyone explain how a default value in mutable map works?</p>
  
  <pre><code>scala&gt; val mmap = mutable.Map[String, mutable.Set[String]]().withDefaultValue{mutable.Set[String]()}
  mmap: scala.collection.mutable.Map[String,scala.collection.mutable.Set[String]] = Map()
  
  scala&gt; mmap("a") += "b"
  res1: scala.collection.mutable.Set[String] = Set(b)
  </code></pre>
  
  <p>Map is empty, no keys.</p>
  
  <pre><code>scala&gt; mmap
  res2: scala.collection.mutable.Map[String,scala.collection.mutable.Set[String]] = Map()
  </code></pre>
  
  <p>But the key I just tried to edit is showing data.</p>
  
  <pre><code>scala&gt; mmap("a")
  res3: scala.collection.mutable.Set[String] = Set(b)
  </code></pre>
  
  <p>Why is <code>res2</code> an empty map but <code>mmap("a")</code> have a value?</p>