Add return_model to bulk_upsert

[xavierdutreilh]

bulk_insert accepts a return_model parameter to be set to True if the result to be returned should be a list of model instances (and not a list of dictionaries). However, the two bulk_upsert methods defined in the managers module do not accept this parameter. So, the result is always a list of dictionaries and it is not possible to request a list of model instances.

As a result, this pull request intends to change that by exposing a return_model parameter on the bulk_upsert methods and pass it all the way to bulk_insert.