Node - PayPal checksum creation function doesn't work

Hey guys,

I fixed a bug where you couldn't create a checksum using the paymill wrapper for nodejs. 

In the "ChecksumService.prototype.create" function.

I'm wondering whether this thing had ever worked? Can't imagine that I'm the first ever noticing it?!
Anyway I'll provide a pull request so I don't need to monkey patch your wrapper. 

Best,
Flo