BaseCloudEvent.getExtensionNames() breaks cloud event encapsulation

please replace

@Override
public Set<String> getExtensionNames() {
    return this.extensions.keySet();
}

with something, like

@Override
public Set<String> getExtensionNames() {
    return Collections.unmodifiableSet (this.extensions.keySet());
}

to avoid breaking encapsulation.

More details can be found here

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#keySet--
Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.