CRM-20527 batch data entry does not retain currency

to reproduce:

1. visit the contribution bulk entry profile config and add the currency field (if not already there)
2. enable multiple currencies on your site
3. create a new data entry batch
4. enter a contribution using a non-default currency
5. click save and continue later
6. re-enter the batch entry screen

note that the previously entered contrib has reverted to the default currency. it seems that if you close the batch the values are retained – but saving and continuing later loses the value.