Add failling test for coerce

[rreusser]

This PR adds a failing test for Lib.coerce. It's pretty short:

var attrs = { 
  x: {
    valType: 'info_array',
    freeLength: true,
    items: [
      { valType: 'any' },
      { valType: 'any' },
      { valType: 'any' }
    ]   
  }
};
out = coerce({x: [[], 10]}, {}, attrs, 'x');
expect(out).toEqual([[], 10]);

It fails because the output is equal to [undefined, 10] instead of [[], 10].

The error goes away if I remove the third valType: 'any', suggesting that freeLength doesn't quite do what it sounds like it does. freeLength has no effect either way, but I've included it since I'd expect it to help. It seems it's refusing to set empty arrays, but I'm not sure why.

[rreusser]

Found the source of the problem: https://github.com/plotly/plotly.js/blob/master/src/lib/nested_property.js#L243

The solution is not straightforward. The result is that you can't actually pass an empty array through plotly. That makes a pause button a bit awkward.

[rreusser]

Solution: The cause of this is that Lib.nestedProperty is written to aggressively clean up after itself. This touches a lot of things so is best not modified. For a pause button where an affirmative empty array [] is meaningful and does not indicate the item should be deleted, it seems the best solution, contingent upon #1121, is to require [null] for pausing instead of [].