need not send message to server when set undefined value

now there is a problem when set undefined or null value, for example:

```
client.set('key', undefined, function (err) {
  console.log(err);
  client.get('key', function (err, item) {
    console.log(typeof item, item);
  });
});
``` 

result:
```
null
string undefined
```
the datatype is changed (undefined changed to string), and if value is invalid , client need not send message to server.