private void doLoaderPut(final RefDataLoader refDataLoader,
                             final MapDefinition mapDefinition,
                             final String key,
                             final RefDataValue refDataValue) {
        writeValue(refDataValue);
        refDataLoader.put(mapDefinition, key, stagingValueOutputStream);
    }
Make sure that it copes with a map not being in an early stream but is in a later one.
stroom-app/src/test/java/stroom/pipeline/refdata/TestReferenceData.java