Reset velocity and constraint jAcc when body type changes to DYNAMIC

[viblo]

A fix for #238

[slembcke]

Another minor issue to point out with this is that if you are adding constraints between non-dynamic bodies, they don't do anything except consume CPU time generating unused NaNs. The client code already needs to be adding and tracking a list of constraints somewhere, all they need to do to fix this is to remove the constraints when making the body static.

I don't doubt that this patch works, but I think I need some more convincing that adding this workaround is worth it as, again this is the first time in 15 years I've heard of someone wanting this.