Made DBCollection.findOne overloads that take the _id value go through the same code path as the other findOne methods.

 This fixes a bug where the read preference is not being correctly applied when connected to a mongos

 JAVA-1473

Author: jyemin

src/main/com/mongodb/DBCollection.java

@@ -378,8 +378,7 @@ public DBObject findOne( Object obj ){
      * @mongodb.driver.manual tutorial/query-documents/ Query
      */
     public DBObject findOne( Object obj, DBObject fields ){
-        Iterator<DBObject> iterator = find(new BasicDBObject("_id", obj), fields, 0, -1, 0, getOptions(), getReadPreference(), getDecoder());
-        return (iterator.hasNext() ? iterator.next() : null);
+        return findOne(new BasicDBObject("_id", obj), fields);
     }
 
     /**