PR: https://github.com/mongodb/mongo-java-driver/pull/416

Right now, there is no way to serialize/deserialize a collection of abstract class or interface correctly, even though BsonDiscriminator annotation is added to all relevant classes (i.e. both parent class and all sub-classes). This patch fixes this problem by using the codec of the actual class of the value during encoding instead of the declared class. The decoding already uses a similar process.