`axis.showline` does not work with `trace.zorder`

[emilykl]

Codepen: https://codepen.io/emilykl-code/pen/ZYEEXvw?editors=1111

Axis lines are not drawn when one of the traces on the plot has a zorder property.

Figure definition:

Plotly.newPlot(
  "div1", 
  [
    {
      x: [11, 12, 13, 14, 15],
      y: [9, 10, 12, 16, 24],
      zorder: 2,
    },
    {
      x: [15, 14, 13, 12, 11],
      y: [9, 10, 12, 16, 24],
    }
  ], 
  {
    title: { text: "A Plotly Chart" },
    xaxis: { showline: true },
    yaxis: { showline: true },
  }
);

Expected (axes lines visible):
(note -- blue trace should be in front)

Actual: