SET with NX flag does not work as expected

[przpl]

Example code

 this._client.SET("key", "value", "NX", (err, reply) => {
    console.log(reply); // should print "OK" only on the first call
});

NX is "Only set the key if it does not already exist" flag.

On real Redis we will get "OK" on the first call and null on the second, which is expected as the value was set at the first time and cannot be overwritten. Redis-mock will respond with "OK" on the first call, second and any other afterward which is not correct behavior.