DATAMONGO-2021 - Use getObjectId() instead of getFilename() for opening the GridFS download stream.

Using the file name leads to duplicate resource streams as file names are not unique therefore we're using the file's ObjectId to lookup the file content.

Original pull request: #581.

Author: nhh

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/gridfs/GridFsTemplate.java

@@ -44,7 +44,7 @@
 
 /**
  * {@link GridFsOperations} implementation to store content into MongoDB GridFS.
- * 
+ *
  * @author Oliver Gierke
  * @author Philipp Schneider
  * @author Thomas Darimont
@@ -62,7 +62,7 @@ public class GridFsTemplate implements GridFsOperations, ResourcePatternResolver
 
 	/**
 	 * Creates a new {@link GridFsTemplate} using the given {@link MongoDbFactory} and {@link MongoConverter}.
-	 * 
+	 *
 	 * @param dbFactory must not be {@literal null}.
 	 * @param converter must not be {@literal null}.
 	 */
@@ -72,7 +72,7 @@ public GridFsTemplate(MongoDbFactory dbFactory, MongoConverter converter) {
 
 	/**
 	 * Creates a new {@link GridFsTemplate} using the given {@link MongoDbFactory} and {@link MongoConverter}.
-	 * 
+	 *
 	 * @param dbFactory must not be {@literal null}.
 	 * @param converter must not be {@literal null}.
 	 * @param bucket