@Override
    public RefDataValueProxy merge(final RefDataValueProxy additionalProxy) {
        return merge(this, additionalProxy);
    }
Merge the two proxies into one combined one that will use each contained proxy to try to get a value.
Argument order is CRITICAL to ensure lookups are done in the correct order.

@param existingRefDataValueProxy The existing
@param newRefDataValueProxy
@return
stroom-pipeline/src/main/java/stroom/pipeline/refdata/store/MultiRefDataValueProxy.java