Default spring config is shared yet mutable

[pingpongboss]

    SpringSystem system = SpringSystem.create();
    Spring a = system.createSpring();
    Spring b = system.createSpring();

    a.getSpringConfig().tension = 123;
    a.getSpringConfig().friction = 456;

    Log.e("REBOUND", "t: " + b.getSpringConfig().tension);
    Log.e("REBOUND", "f: " + b.getSpringConfig().friction);

In this example, both a and b share the same default SpringConfig instance. This becomes an issue when code tries to mutate the SpringConfig on a. We find that the SpringConfig on b is accidentally mutated as well.