Avoid double call of fetch method in DefaultReferenceResolver.

This commit fixes an issue where the fetch method is called twice when looking up singe value references.

Resolves: #3918
Original Pull Request: #3919

Author: TheHett

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DefaultReferenceResolver.java

@@ -19,6 +19,7 @@
 
 import java.util.Collections;
 
+import org.bson.Document;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.data.mongodb.core.mapping.DBRef;
 import org.springframework.data.mongodb.core.mapping.DocumentReference;
@@ -32,6 +33,7 @@
  *
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Anton Buzdalkin
  * @since 3.3
  */
 public class DefaultReferenceResolver implements ReferenceResolver {
@@ -41,8 +43,8 @@ public class DefaultReferenceResolver implements ReferenceResolver {
 
 	private final LookupFunction collectionLookupFunction = (filter, ctx) -> getReferenceLoader().fetchMany(filter, ctx);
 	private final LookupFunction singleValueLookupFunction = (filter, ctx) -> {
-		Object target = getReferenceLoader().fetchOne(filter, ctx);
-		return target == null ? Collections.emptyList() : Collections.singleton(getReferenceLoader().fetchOne(filter, ctx));
+		Document target = getReferenceLoader().fetchOne(filter, ctx);
+		return target == null ? Collections.emptyList() : Collections.singleton(target);
 	};
 
 	/**