#2 saving a class without attributes leads to crash.

[Willem van den Ende]

If there is a class without persistent attributes, it
is not saved, when trying Mirage.sync(), mirage crashes.

Solution: write an integration test that saves the
class with no attributes more than once, reload the
class in a fresh Java VM, and try to sync it again.

[Willem van den Ende]

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Ruby integration test added to 1.0, fixed in 1.0. With
acceptance test also fixable in mirage_performance