Convert fen param to string before split

Hi, I'm using Chess.js and I think that this little fix will be appreciated

I suggest just to convert the fen parameter to a string before try to split it, to avoid errors when trying to split something that hasn't split method.

Current behavior: 
![screen shot 2014-06-21 at 3 32 33 am](https://cloud.githubusercontent.com/assets/736728/3348055/9e1c57ec-f90e-11e3-9cdc-93bb12b6cb92.png)

With this fix:
![screen shot 2014-06-21 at 3 39 58 am](https://cloud.githubusercontent.com/assets/736728/3348060/e96dee7c-f90e-11e3-860f-53ec290fcdd6.png)

Best