title: Updating one key in list of maps using ImmutableJS
body: <p>Docs for immutable.js, lack descriptive examples. 
  Could somebody kindly explain, how I could perform the following in ImmutableJS:</p>
  
  <pre><code>function isOdd (v) {
      return v % 2 === 0
  }
  
  var collection = [{a: 1, b: 2}, {a: 3, b: 7}, {a: 5, b: 6}];
  collection.map(item =&gt; {
   if (isOdd(item.b))  {
     item.a = item.a * 2;
   }
   return item;
  })
  </code></pre>
  
  <p>Any help is highly appreciated.</p>