fixed bug with debug mode

using `assert msg.pop(0) == ''` shows different behaviours depending on whether you run the code with `python -O` or simply with `python` (differently said, if you run in debug mode or not...).

In particular when not in debug mode with `worker.recv()` you get a `request` obejct starting with an empty element.

examples/Python/mdwrkapi.py

@@ -132,7 +132,8 @@ def recv(self, reply=None):
  # up to a null part, but for now, just save one...
  self.reply_to = msg.pop(0)
  # pop empty
- assert msg.pop(0) == ''
+ empty = msg.pop(0)
+ assert empty == ''
 
  return msg # We have a request to process
  elif command == MDP.W_HEARTBEAT: