Specifying a custom icon with icons\list_icon doesn't seem to work

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I've been trying to specify a custom icon for android as per docs: list_icon_example

However the icon is always the default.

I've been experimenting with the following example:

let pushObject = {
    audience:"ALL" , // or specify your target audience
    device_types: ['android'],
    notification: {
        "alert": "Goal 🚨"
    },
    "message": {
    "title": "This is Great 😍",
    "content_type": "text/html",
    "icons": {
      "list_icon": "http://static.thenounproject.com/png/3061218-200.png"
    }
  }
};