Ignored fields list for copyToRealmOrUpdate

User case from SO: http://stackoverflow.com/questions/35244823/realm-for-android-how-to-sync-server-data-to-realm-database

If one field is stored locally, it would be convenient to ignore the local field while updating the existing RealmObject.

API proposal:

public <E extends RealmObject> E copyToRealmOrUpdate(E object, String... ignoredFields)