Transaction.updatable_fields will force _update to raise a KeyError

[AndreiHondrari]

On this line:

paymill-python/paymill/models/transaction.py

Line 76 in 67e4708

return 'description'

a string is returned, which is being iterated over here:

paymill-python/paymill/services/paymill_service.py

Line 38 in 67e4708

for u in obj.updatable_fields():

but it should have been a tuple like the other models. I guess a missing comma will do it.