Physics speed doubles when ran a second time

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What steps will reproduce the problem?
1. Create a physics simulation
2. Do something else (like going to level selection
3. Run the same physics simulation again

What is the expected output? What do you see instead?
Everything is moving twice the speed

What version of the product are you using? On what operating system?
2.1.a.3 Windows Vista

Please provide any additional information below.
On some tutorials like https://www.scirra.com/manual/98/physics people warned 
about this and told to put Set Stepping Mode to "fixed". But I can't find such 
a property in Box2dWeb.Has this been implemented? If not, is there some other 
way of keeping the speed fixed?

Original issue reported on code.google.com by oneminut...@gmail.com on 14 Oct 2013 at 2:47