systemjs load css suport url(image/test.svg) without quotes in url

[Sunkey]

Demonstration

systemjs load css replace url content not support like "url(image/test.svg)" without quotes in url.

Code Sandbox:
system.js code:

background-image: url(images/test.svg) convert to background-image: url("undefined${cssDir}/images/test.svgundefined"), cause resource load error

Expected Behavior

systemjs load css support url without quote

Actual Behavior

load error