subscription cannot historize for document other than Invoice or Sale order

When a subscription for a document other than "Invoice" or "Sale Order" runs, the cron job fails on trying
to create the subscription history.

Steps to reproduce:

* clean trunk code + database with project and subscription modules installed
* under tools / configuration / recurring events / document types create document type "Task" (project.task)
* under tools / Recurring Events / Subscriptions create subscription for a task and press "process"

Wait for the cron job to run , you will see following error in the console:

    _('The value "%s" for the field "%s.%s" is not in the selection') % (value, self._table, field))
except_orm: ('ValidateError', u'The value "project.task,64" for the field "subscription_subscription_history.document_id" is not in the selection')

Possible fix:

in the class subscription_subscription_history (in subscription/subscription.py line 151) change the selection attribute of the document_id field to _get_document_types (as in subscription.subscription field doc_source).