Correct center in constructor

Author: samme

src/physics/arcade/Body.js

@@ -281,7 +281,7 @@ var Body = new Class({
          * @type {Phaser.Math.Vector2}
          * @since 3.0.0
          */
-        this.center = new Vector2(gameObject.x + this.halfWidth, gameObject.y + this.halfHeight);
+        this.center = new Vector2(this.position.x + this.halfWidth, this.position.y + this.halfHeight);
 
         /**
          * The Body's velocity, in pixels per second.