optimize aggregate()

The aggregate method can get BsonDocument(s) directly instead getting the iterator of Document(s) first and then convert them back to the BsonDocument(s).

https://www.eclipse.org/lists/jnosql-dev/msg00642.html

Author: redmitry

jnosql-mongodb/src/main/java/org/eclipse/jnosql/databases/mongodb/communication/MongoDBDocumentManager.java

@@ -208,10 +208,9 @@ public long delete(String collectionName, Bson filter) {
     public Stream<Map<String, BsonValue>> aggregate(String collectionName, List<Bson> pipeline) {
         Objects.requireNonNull(pipeline, "filter is required");
         Objects.requireNonNull(collectionName, "collectionName is required");
-        MongoCollection<Document> collection = mongoDatabase.getCollection(collectionName);
-        AggregateIterable<Document> aggregate = collection.aggregate(pipeline);
-        return stream(aggregate.spliterator(), false)
-                .map(Document::toBsonDocument);
+        MongoCollection<BsonDocument> collection = mongoDatabase.getCollection(collectionName, BsonDocument.class);
+        AggregateIterable aggregate = collection.aggregate(pipeline);
+        return stream(aggregate.spliterator(), false);
     }
 
     /**