LibWeb: Make CanvasRenderingContext2D::fill's fillRule argument optional

As defined by the specification (and used by the website i am testing):
interface mixin CanvasDrawPath {
  undefined fill(optional CanvasFillRule fillRule = "nonzero");
}

Userland/Libraries/LibWeb/HTML/CanvasRenderingContext2D.idl

@@ -10,7 +10,7 @@ interface CanvasRenderingContext2D {
 
  undefined beginPath();
  undefined closePath();
- undefined fill(DOMString fillRule);
+ undefined fill(optional DOMString fillRule = "nonzero");
  undefined stroke();
  undefined moveTo(double x, double y);
  undefined lineTo(double x, double y);