supervisor version 4.0.0-dev: fix AttributeError: 'unicode' object has no attribute 'more'

The following is error trace:

2014-07-19 17:48:19,877 TRAC Web interface error:Traceback (most recent call last):
  File "/home/son/Study/supervisor/supervisor/web.py", line 54, in more
    return self.sendresponse(response)
  File "/home/son/Study/supervisor/supervisor/web.py", line 139, in sendresponse
    self.request.log
  File "/home/son/Study/supervisor/supervisor/medusa/asynchat_25.py", line 164, in push_with_producer
    self.initiate_send()
  File "/home/son/Study/supervisor/supervisor/medusa/asynchat_25.py", line 214, in initiate_send
    self.refill_buffer()
  File "/home/son/Study/supervisor/supervisor/http.py", line 362, in refill_buffer
    data = p.more()
  File "/home/son/Study/supervisor/supervisor/medusa/producers.py", line 174, in more
    data = self.producer.more()
  File "/home/son/Study/supervisor/supervisor/medusa/producers.py", line 198, in more
    result = self.producer.more()
  File "/home/son/Study/supervisor/supervisor/medusa/producers.py", line 151, in more
    d = p.more()
AttributeError: 'unicode' object has no attribute 'more'
2014-07-19 17:48:19,878 TRAC 0.0.0.0:Server Error: <type 'exceptions.AttributeError'>, 'unicode' object has no attribute 'more': file: /home/son/Study/supervisor/supervisor/medusa/producers.py line: 151
2014-07-19 17:48:19,878 BLAT write event caused by <socket._socketobject object at 0x7f508c105a60>
error: uncaptured python exception, closing channel <supervisor.http.deferring_http_channel connected 127.0.0.1:55613 at 0x7f508c0ffab8 channel#: 2 requests:1> (<type 'exceptions.AttributeError'>:'unicode' object has no attribute 'more' [/home/son/Study/supervisor/supervisor/supervisord.py|runforever|259] [/home/son/Study/supervisor/supervisor/medusa/asyncore_25.py|handle_write_event|398] [/home/son/Study/supervisor/supervisor/medusa/asynchat_25.py|handle_write|153] [/home/son/Study/supervisor/supervisor/medusa/asynchat_25.py|initiate_send|214] [/home/son/Study/supervisor/supervisor/http.py|refill_buffer|362] [/home/son/Study/supervisor/supervisor/medusa/producers.py|more|174] [/home/son/Study/supervisor/supervisor/medusa/producers.py|more|198] [/home/son/Study/supervisor/supervisor/medusa/producers.py|more|151])