Change AssumptionViolatedException to not set the cause to null; fixes issue #494

The two-argument `String, Throwable` constructor seems the right place indeed, as there it is certain we have a throwable.

But I'm a bit puzzled about the status of the 4-argument constructor.
If it can be used, why is there no javadoc?
If it should not be used from outside the class why isn't it private? (Or documented?)

The same for the four-argument constructor from the (non-deprecated) API counterpart `org.junit.AssumptionViolatedException`