Fixed AttributeError in _threaded_return caused when execution modules yield dicts.

/var/log/salt/minion:

[salt.minion                                 ][WARNING ] The minion
function caused an exception
Traceback (most recent call last):
  File "/usr/lib/python2.6/site-packages/salt/minion.py", line 803, in
  _thread_return
      iret.update(single)