CSHARP-553: change the order of $readPreference and $query from the documented order to the order that actually works.

Author: rstam

Driver/Core/MongoCursorEnumerator.cs

@@ -366,8 +366,8 @@ private IMongoQuery WrapQuery()
                 var query = (_cursor.Query == null) ? (BsonValue)new BsonDocument() : BsonDocumentWrapper.Create(_cursor.Query);
                 var wrappedQuery = new QueryDocument
                 {
+                    { "$query", query },
                     { "$readPreference", formattedReadPreference, formattedReadPreference != null }, // only if sending query to a mongos
-                    { "$query", query }
                 };
                 wrappedQuery.Merge(_cursor.Options);
                 return wrappedQuery;