Fix querystring.stringify( {array: []} ). Now it returns "array="

I found strange behavior of ```QueryString.stringify( {array: []} ) === ""```
Other strange cases usually returns something like ```"key="```
For example:
```javascript
QueryString.stringify( {a: undefined, b: null, c: NaN, d: ""} ); 
// "a=&b=&c=&d="
```
So for conveniece it should be:
```javascript
QueryString.stringify( {array: []} )
// "array="
```