[decorators] contentIconPath doesn't follow URLs like gutterIconPath

[drew-wallace]

• VSCode Version: 1.4.0
• OS Version: OSX 10.11.6

Steps to Reproduce:

1. Create a text decoration with DecorationRenderOptions

{
    "gutterIconPath": "http://image.flaticon.com/icons/svg/188/188918.svg",
    "before": {
        "contentIconPath": "http://image.flaticon.com/icons/svg/188/188918.svg",
        "height": "14px",
        "width": "14px"
    }
}

2. Set the decoration and notice the gutter icon appears, but the inline icon is missing