[Bullet] Setting 3D gravity vector during runtime disables it instead

[Cadmik]

Godot version:
3.1.1

OS/device including version:
Ubuntu 18.04

Issue description:
Calling PhysicsServer.area_set_param to update a world's gravity vector seems to disregard the provided value, instead setting it to 0. After some digging, I found this behaviour consistent with commit d250ade (link kindly provided by a user in IRC), and I can confirm that the gravity vector behaves as expected after reverting the commit. However I am not sure if doing so reintroduced whatever bug it was fixing, thus this is not a pull request.

Steps to reproduce:

1. Add Camera and RigidBody (any shape works, needs some visible geometry as child node).
2. Extend RigidBody node with a script, and set world gravity vector within _process (to anything other than 0).
3. Play the project.

Minimal reproduction project:
NoGravity.zip