Don’t test self-collisions

A useful technique in Flixel is to have a group of objects, and perform collision between them like this:

var actors = game.add.group();
...
game.physics.collide(actors, actors);
This resolves collisions within the group. Phaser currently collides objects within the group against themselves, which limits the usefulness of this approach.

I've attached a simple fix, although I don't know if the performance cost is worth it.

I also don't know if thats sufficient when you can have TileLayers within groups, for example. (I'm not familiar with using Tilemaps)