Python Lists translated to javascript objects in widgets

[jasongrout]

From @ChakriCherukuri:

I just got the latest code from master and I am trying to create my custom widgets. One thing I noticed is that List() traitlet is no longer being mapped to an array on JS side. It is now mapped
into an object something like {0: v1, 1: v2, 2: v3, ...} as opposed to an array like [v1, v2, v3, ...]. It would be great if someone can look into this issue.

From @jasongrout: It looks like the javascript unpacker should be checking to see if the value is $.isArray before checking to see if it is an object here and handling arrays by creating a javascript array: https://github.com/ipython/ipython/blob/master/IPython/html/static/notebook/js/widgets/widget.js#L225.

Sources: #4374 (comment) (initial bug report), #4374 (comment) and #4374 (comment) track down the problem and outline a fix.