Handling of history enabled models
When using proteus on history enabled models like party.party and
party.address provided by the account_invoice_history module[1] it is needed
to set explicit a datetime context, like::
>>> with config.set_context({'_datetime': invoice.open_date}):
... invoice = Invoice(invoice.id)
... invoice.party.name
... invoice.invoice_address.street
It would be simpler and more consitent with the Tryton gtk client, when the
context is set automatically, using the _datetime provided by the field[2].
[1] http://hg.tryton.org/modules/account_invoice_history/file/f89ffdc4a28e/party.py
[2] http://hg.tryton.org/modules/account_invoice_history/file/f89ffdc4a28e/invoice.py#l19