Fixed two bugs in test model's __str__ method:  * it referenced TranslationDoesNotExist exception without    importing it first,  * it did not use str() to convert all the data fields to string.
Fixed two bugs in test model's __str__ method:
 * it referenced TranslationDoesNotExist exception without
   importing it first,
 * it did not use str() to convert all the data fields to string.

DataMarket/multilingual,DataMarket/multilingual,DataMarket/multilingual