stock_picking.action_invoice_create: calling by xmlrpc raises TypeError

Hello,

When the action_invoice_create method of stock.picking is called via XMLRPC, an exception is reported, because that method returns a dictionary with non-string keys:

      *********** OpenERP Server ERROR ***********
      dictionary key must be string
      Traceback (most recent call last):
        File "/home/afayolle/work/alpes_dev/openerp_buildout/src/server/openerp/wsgi/core.py", line 80, in xmlrpc_return
          response = xmlrpclib.dumps((result,), methodresponse=1, allow_none=False, encoding=None)
        File "/usr/lib/python2.7/xmlrpclib.py", line 1085, in dumps
          data = m.dumps(params)
        File "/usr/lib/python2.7/xmlrpclib.py", line 632, in dumps
          dump(v, write)
        File "/usr/lib/python2.7/xmlrpclib.py", line 654, in __dump
          f(self, value, write)
        File "/usr/lib/python2.7/xmlrpclib.py", line 733, in dump_struct
          raise TypeError, "dictionary key must be string"
      TypeError: dictionary key must be string
      ********************************************

The action is performed correctly, though. And the web client probably ignores the errors. But it is very disturbing when scripting OpenERP to run tests.

I'm reporting this on openobject-addons stable because this is where I observed the issue. I understand that changing this in stable is not going to be possible, but I would appreciate if a fix could be made for this in openobject server trunk.

A simple workaround could be handling TypeError in xmlrpc_return (core.py) when calling xmlrpclib(dumps(...)) and issuing a warning in the server log about this before returning a normal status.

Thanks for your time.