Avoid obtaining mapped sort multiple times for mapReduce.

Apply already mapped sort for map reduce instead of running the source document through the mapping layer again.

Closes: #3960

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoTemplate.java

@@ -1848,7 +1848,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		Document mappedSort = getMappedSortObject(query, domainType);
 		if (mappedSort != null && !mappedSort.isEmpty()) {
-			mapReduce = mapReduce.sort(getMappedSortObject(query, domainType));
+			mapReduce = mapReduce.sort(mappedSort);
 		}
 
 		mapReduce = mapReduce