@Override
    public JsonSerializer<Object> getUnknownTypeSerializer(final Class<?> aClass) {
        return unknownTypeSerializer;
    }
Implementation of the {@code DefaultSerializerProvider} for Jackson that uses the {@code ToStringSerializer} for
unknown types.

@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/io/graphson/GraphSONSerializerProvider.java