if values is undefined or null use original query function

Hi,

thanks for this monkey patch!

The current code fails for:

                        pg
                        .query(db_query, null, function(err, result) {
                                callback(err, result.rows);
                            });

with

     Uncaught TypeError: Object.keys called on non-object
      at Function.keys (native)
      at numericFromNamed (/Users/ca/devel/git/ts_backend/node_modules/node-postgres-named/main.js:6:31)
      at patchedQuery [as query] (/Users/ca/devel/git/ts_backend/node_modules/node-postgres-named/main.js:55:29)

This pull request checks whether the second parameter is null or undefined (if there are 3 parameters) and uses the original function in that case, as well.