setMassProps didn't adjust gravity force, this might impact some projects

See also 
http://bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=4835
and
http://code.google.com/p/bullet/issues/detail?id=364

src/BulletDynamics/Dynamics/btRigidBody.cpp

@@ -239,6 +239,9 @@ void btRigidBody::setMassProps(btScalar mass, const btVector3& inertia)
 		m_collisionFlags &= (~btCollisionObject::CF_STATIC_OBJECT);
 		m_inverseMass = btScalar(1.0) / mass;
 	}
+
+	//Fg = m * a
+	m_gravity = mass * m_gravity_acceleration;
 	
 	m_invInertiaLocal.setValue(inertia.x() != btScalar(0.0) ? btScalar(1.0) / inertia.x(): btScalar(0.0),
 				   inertia.y() != btScalar(0.0) ? btScalar(1.0) / inertia.y(): btScalar(0.0),