in hmset, use JSON.stringify for objects with objects as values

Hi @DTrejo 

When I tried to put an obj to hash using hmset, 
```javascript
var obj = {a:1, b:2, c:3, d:{A:1, B:2, C:3}};
console.log(obj);

redis
  .hmset('test-124', obj, function(err, reply){
    redis.hgetall('test-124', function(err, reply2){
      console.log(reply2);
    });
});
```

I found the following output.
```javascript
{ a: 1, b: 2, c: 3, d: { A: 1, B: 2, C: 3 } }
{ a: '1', b: '2', c: '3', d: '[object Object]' }
```
Does this mean that I cannot store a nested obj in redis?



However, according to the master code, I found the following:
```javascript
if (typeof args[1][key] !== "string") {
   var err = new Error("hmset expected value to be a string", key, ":", args[1][key]);
```

This means using my obj in hmset would result in an error, however it does not, which I do not understand ...

What I want, is to be able to store the object using JSON.stringify. Can we bring up a pull request for this?