private static DocumentUpdate roundtripSerialize(DocumentUpdate update) {
        GrowableByteBuffer buffer = serializeDocumentUpdate(update);
        return deserializeDocumentUpdate(buffer);
    }
Tests roundtrip serialization (JSON -> DocumentUpdate -> Buffer -> DocumentUpdate -> JSON) of document updates.

@author Vegard Sjonfjell
document/src/test/java/com/yahoo/document/json/DocumentUpdateJsonSerializerTest.java