Allow client to control ignorable properties in DefaultToSoyDataConverter

https://github.com/mati1979/spring-soy-view/issues/46
Good point. I'll make the change and resubmit it. Unmodifiable seems to be a bit overkill because the only place this is referenced is in the private class and I'm not sure if unmodifiable is going to give us any concurrency benefits. That said, I don't feel strongly against it either.