aoa_to_sheet() crashes browser if you pass an array of objects

I know you're not supposed to do this, but if you pass an array of objects into `aoa_to_sheet()` it gets stuck in an infinite loop.

E.g. 
```
XLSX.utils.aoa_to_sheet([{"hello": "world"}]);
// RIP your chrome tab X_X
```
[JSfiddle](https://jsfiddle.net/j9aswgL8/8/)

Might be good to throw an exception in this situation ;)