Allow client to control ignorable properties in DefaultToSoyDataConverter

https://github.com/mati1979/spring-soy-view/issues/46
Cheers.  You are right, the reference is not exposed, so the risk is low.  Though, generally unmodifiableList(...) is also used along with final to document that a field should not change (and not just that it can't).  If you want to use the Guava Immutable collection type, that's cool too.