context.fillStyle should be a string

The value we set context.fillStyle to should always be a string. This commit fixes a bug in circlestyle.js where we did set context.fillStyle to an array instead of a string.

src/ol/style/circlestyle.js

@@ -405,7 +405,7 @@ ol.style.Circle.prototype.drawHitDetectionCanvas_ =
       renderOptions.size / 2, renderOptions.size / 2,
       this.radius_, 0, 2 * Math.PI, true);
 
-  context.fillStyle = ol.render.canvas.defaultFillStyle;
+  context.fillStyle = ol.color.asString(ol.render.canvas.defaultFillStyle);
   context.fill();
   if (!goog.isNull(this.stroke_)) {
     context.strokeStyle = renderOptions.strokeStyle;