Product.find_by does not consider the parameters

[neywen]

as a complement of my previous issue : #5

I tested the example provided with the Moltin documentation :

product = moltin_instance.Product.find_by({'slug': 'decorative-hedgehogs'})

and it returns the entire list of products, in the limit of 10 products (I suppose this is the default limit setup in the dashboard settings).

[neywen]

I sent an PR to fix the bug. #8