Can't set a default origin for stock pickings when duplicating from Python code

[gurneyalex]

https://bugs.launchpad.net/ocb-addons/+bug/1098557

In the Python code of custom addons, it would sometimes be useful to be able to copy a picking while setting the origin field.
For example:

        defaults={'origin': 'test', 'type': 'internal'}
        id = self.pool.get('stock.picking').copy(cr, uid, 1, defaults, context={})

However, in v6.0 and up to the current trunk, the method copy() of stock.picking resets the field "origin" as soon as no default name is set.
I suggest the origin be reset only if it's not passed in the dict of default values.

[mart-e]

Patch merged at 8af9fdf.
Regards