title: immutable.js get keys from map/hash
body: <p>I want to retrieve keys() from the following Immutable Map:</p>
  
  <pre><code>var map = Immutable.fromJS({"firstKey": null, "secondKey": null });
  console.log(JSON.stringify(map.keys()));
  </code></pre>
  
  <p>I would expect the output:</p>
  
  <pre><code>["firstKey", "secondKey"]
  </code></pre>
  
  <p>However this outputs:</p>
  
  <pre><code>{"_type":0,"_stack":{"node":{"ownerID":{},"entries":[["firstKey",null],["secondKey",null]]},"index":0}}
  </code></pre>
  
  <p>How to do it properly? </p>
  
  <p>JSFiddle link: <a href="https://jsfiddle.net/o04btr3j/57/" rel="noreferrer">https://jsfiddle.net/o04btr3j/57/</a></p>