OAF-store-graph mdstores: save them in text format

dhp-workflows/dhp-aggregation/src/main/scala/eu/dnetlib/dhp/collection/CollectionUtils.scala

@@ -50,14 +50,16 @@ object CollectionUtils {
 
   def saveDataset(dataset: Dataset[Oaf], targetPath: String): Unit = {
     implicit val resultEncoder: Encoder[Oaf] = Encoders.kryo(classOf[Oaf])
+    val mapper = new ObjectMapper
 
     dataset
       .flatMap(i => CollectionUtils.fixRelations(i))
       .filter(i => i != null)
+      .map(r => mapper.writeValueAsString(r))(Encoders.STRING)
       .write
       .mode(SaveMode.Overwrite)
       .option("compression", "gzip")
-      .json(targetPath)
+      .text(targetPath)
   }
 
 }