plotly.js bgcolor validation exception

plotly.js validation throws exception for valid `paper_bgcolor` and `plot_bgcolor`.

by tbitai

HTML

<script src="https://cdn.plot.ly/plotly-1.38.3.js"></script>
<div id="plotly"></div>

CSS

#ploty {
 width: 100%;
 height: 100%;
}

JavaScript

let data = [{
	x: [0, 1, 2],
  y: [0, 1, 4]
}];
let layout = {
	paper_bgcolor: "grey",
  plot_bgcolor: "yellow"
};

// This will throw an exception. But if you uncomment 
// it, correct layout will be plotted below.
// The same is true for either just `paper_bgcolor`
// or `plot_bgcolor` alone.
Plotly.validate(data, layout);

Plotly.plot('plotly', data, layout);