private void registerPredicateDocumentType() {
        DocumentType x = new DocumentType("testpredicate");
        DataType d = DataType.PREDICATE;
        x.addField(new Field("actualpredicate", d));
        types.registerDocumentType(x);
    }
Functional tests for com.yahoo.document.json.JsonWriter.

@author Steinar Knutsen
document/src/test/java/com/yahoo/document/json/JsonWriterTestCase.java