PhysicsBody2D fails to generate collision shapes when the shape is added to a PhysicsBody2D instanced subscene

[HeartoLazor]

Operating system or device - Godot version:
Windows 10
Godot 2.1.1 stable

Issue description (what happened, and what was expected):
PhysicsBody2D fails to generate collision shapes when the shape is added to a PhysicsBody2D instanced subscene.
Also creating a subscene with the Shape, Instance it and edit the shape from the Collision shape with editable children doesn't work, the Collision Shape retains the collision from the instance before the edits.
This seems to be a bug, because the user should expect always a working PhysicsBody2D if they have a children shape. Editable children flag in the subscene doesn't fix the issue.

In my game I want a generic collider with a script which only objetive is to damage enemies or player, the main requirement is this collider should have different shapes, for example a bullet could be a circle shape or a big rectangle, etc. So I created an Area2D subscene with my script without a shape, and my plan is to add custom shapes to subscene instances. That idea failed when the engine failed to create the collision shapes in this case.

Steps to reproduce:

• Create a PhysicsBody2D node (Area2D, RigidBody2d, StaticBody2D, etc)
• Make that PhysicsBody2D a subscene but don't add a Collision Shape
• Instance the subscene and add a CollisionShape to the instance, not the subscene

Link to minimal example project:
Press play and wait for rigid bodies to collide with floor, subscene ones don't collide:
https://dl.dropboxusercontent.com/s/ffi6rapu9co0p7m/custom_collision_shape.zip

[vnen]

Duplicate of #2314.