Declare two fields as final in BsonBinaryReader.Mark

Author: jyemin

bson/src/main/org/bson/BsonBinaryReader.java

@@ -380,8 +380,8 @@ public void reset() {
     }
 
     protected class Mark extends AbstractBsonReader.Mark {
-        private int startPosition;
-        private int size;
+        private final int startPosition;
+        private final int size;
 
         protected Mark() {
             super();