title: Push new object at specific index using immutability helper
body: <p>I have tried to do this:</p>
  
  <pre><code> const rowObj = {key: value};
      const rowIndex = 2;
  
      this.setState({ 
            data: update(this.state.data, 
                { [rowIndex] : { $push : [rowObj] } }
            ) 
      })
  </code></pre>
  
  <p>But, It throw error like this: </p>
  
  <pre><code>Uncaught Error: update(): expected target of $push to be an array; got undefined.
  </code></pre>