InsertAtIndex insert Arrays

[kuserich]

When given an array to insert, insertAtIndex adds the items of the array rather than the array.

Example:

const anArray = [ 'a', 'b', 'c' ];
const input = [ 'x' ];

insertAtIndex( anArray, input );

// => returns [ 'a', 'b', 'c', 'x' ]

I believe that this is a bug and that the result should be [ 'a', 'b', 'c', [ 'x' ].

This will be particularly relevant in case that the input array is an array of arrays such that the value to be added is in fact an array and should be added as an array.

This issue was initially raised in #69 (discussion).