p2 object re-created on system start.

Author: photonstorm

src/physics/Physics.js

@@ -131,15 +131,15 @@ Phaser.Physics.prototype = {
     */
     startSystem: function (system) {
 
-        if (system === Phaser.Physics.ARCADE && this.arcade === null)
+        if (system === Phaser.Physics.ARCADE)
         {
             this.arcade = new Phaser.Physics.Arcade(this.game);
         }
-        else if (system === Phaser.Physics.P2JS && this.p2 === null)
+        else if (system === Phaser.Physics.P2JS)
         {
             this.p2 = new Phaser.Physics.P2(this.game, this.config);
         }
-        if (system === Phaser.Physics.NINJA && this.ninja === null)
+        if (system === Phaser.Physics.NINJA)
         {
             this.ninja = new Phaser.Physics.Ninja(this.game);
         }

src/physics/p2/World.js

@@ -662,8 +662,9 @@ Phaser.Physics.P2.prototype = {
         this.world.off("beginContact", this.beginContactHandler, this);
         this.world.off("endContact", this.endContactHandler, this);
 
-        this.setPostBroadphaseCallback(null);
-        this.setImpactEvents(null);
+        this.postBroadphaseCallback = null;
+        this.callbackContext = null;
+        this.impactCallback = null;
 
         this.collisionGroups = [];
         this._toRemove = [];