[7.0] Merging purchase order will fail when first one has no origin

[Yannick Vaucher @ Camptocamp]

WARNING This affects only OCB

https://code.launchpad.net/~yann-papouin/ocb-addons/7.0-bug-1091268-purchase-merge-no-origin-duplicates/+merge/210169

Introduced a bug:

When first purchase order has no origin, it will raise the following error:

OpenERP Server Error Client Traceback (most recent call last):

    File ".../webclient/addons/web/http.py", line 204, in dispatch
    response["result"] = method(self, **self.params)
    File ".../webclient/addons/web/controllers/main.py", line 1132, in call_button
    action = self._call_kw(req, model, method, args, {})
    File ".../webclient/addons/web/controllers/main.py", line 1120, in _call_kw
    return getattr(req.session.model(model), method)(*args, **kwargs)
    File ".../webclient/addons/web/session.py", line 42, in proxy
    result = self.proxy.execute_kw(self.session._db, self.session._uid, self.session._password, self.model, method, args, kw)
    File ".../webclient/addons/web/session.py", line 30, in proxy_method
    result = self.session.send(self.service_name, method, *args)
    File ".../webclient/addons/web/session.py", line 103, in send
    raise xmlrpclib.Fault(openerp.tools.ustr(e), formatted_info)
    Server Traceback (most recent call last):
    File ".../webclient/addons/web/session.py", line 89, in send
    return openerp.netsvc.dispatch_rpc(service_name, method, args)
    File ".../server/openerp/netsvc.py", line 296, in dispatch_rpc
    result = ExportService.getService(service_name).dispatch(method, params)
    File ".../server/openerp/service/web_services.py", line 626, in dispatch
    res = fn(db, uid, *params)
    File ".../server/openerp/osv/osv.py", line 190, in execute_kw
    return self.execute(db, uid, obj, method, *args, **kw or {})
    File ".../server/openerp/osv/osv.py", line 132, in wrapper
    return f(self, dbname, *args, **kwargs)
    File ".../server/openerp/osv/osv.py", line 199, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
    File ".../server/openerp/osv/osv.py", line 187, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
    File ".../addons/purchase/wizard/purchase_order_group.py", line 71, in merge_orders
    allorders = order_obj.do_merge(cr, uid, context.get('active_ids',[]), context)
    File ".../addons/purchase/purchase.py", line 796, in do_merge
    if not porder.origin in order_infos['origin'] and not order_infos['origin'] in porder.origin: TypeError: argument of type 'bool' is not iterable