isSimple: Rewrite method

*code*
```
function isSimple(obj) {
    return ['Object', 'Function', 'Array'].indexOf({}.toString.call(obj).slice(8, -1)) === -1;
}

[
    1,
    '1',
    true,
    null,
    undefined,
    [1,2,3],
    {},
    function(){}
]
    .forEach(function(type) {
        console.log(type + ' : ' + isSimple(type));
    });
```

*result*
```
1 : true 
1 : true 
true : true 
null : true 
undefined : true 
1,2,3 : false 
[object Object] : false 
function (){} : false
```