Object nesting occurs with repeated agents

``` js
const obj = {
p: {
}
};

const t = proxyClone(obj);

t.p // will go proxyClone
t.p // will go proxyClone again
```

I think you should add this code at
``` js
if (isObject(value) && !override[name] /* Prevent multiple proxyClone */ ) {
value = proxyClone(value);
override[name] = value;
}
```

> Can I submit PR?