Setting borderWidth: 0 does not remove the border, however setting borderWidth:0.01 does.
Setting borderOpacity:0 does not hide the border, however setting borderOpacity:0.01 does.
In both cases it looks like some kind of rounding error.
Checked in Chrome, FFox and Safari on OSX
Did you try using null instead of zero? I noticed that in the examples.
Setting to null should solve this for you. This is a bug with SVG as setting zero still technically sets a border.