Changed the none representation in a more opaque way

src/main/java/org/gcube/data_catalogue/grsf_publish_ws/utils/HelperMethods.java

@@ -72,7 +72,7 @@ public abstract class HelperMethods {
 	private static final String PATH_SEPARATOR = "/";
 	
 	// This key is used in replaceFieldsKey() function to indicate to remove the metadata field
-	private static final String NONE_KEY = "none";
+	private static final String NONE_KEY = "none:none";
 
 	// caches
 	private static CacheInterface<String, String> userEmailCache = new CacheImpl<String, String>(1000 * 60 * 60 * 24);