fix multiplexer, payload should be str

[janx]

I got this error occasionally:

Traceback (most recent call last):
  File "/home/jan/.envs/pyethapp/lib/python2.7/site-packages/gevent/greenlet.py", line 534, in run
    result = self._run(*self.args, **self.kwargs)
  File "/home/jan/projects/opensource/pydevp2p/devp2p/peer.py", line 230, in _run_decoded_packets
    self._handle_packet(self.mux.packet_queue.get())  # get_packet blocks
  File "/home/jan/projects/opensource/pydevp2p/devp2p/peer.py", line 204, in _handle_packet
    protocol.receive_packet(packet)
  File "/home/jan/projects/opensource/pydevp2p/devp2p/protocol.py", line 165, in receive_packet
    cmd(packet)
  File "/home/jan/projects/opensource/pydevp2p/devp2p/protocol.py", line 137, in receive
    instance.receive(proto=self, data=klass.decode_payload(packet.payload))
  File "/home/jan/projects/opensource/pyethapp/pyethapp/eth_protocol.py", line 139, in decode_payload
    for block in rlp.decode_lazy(rlp_data):
  File "/home/jan/.envs/pyethapp/lib/python2.7/site-packages/rlp/lazy.py", line 30, in decode_lazy
    item, end = consume_item_lazy(rlp, 0)
  File "/home/jan/.envs/pyethapp/lib/python2.7/site-packages/rlp/lazy.py", line 55, in consume_item_lazy
    t, l, s = consume_length_prefix(rlp, start)
  File "/home/jan/.envs/pyethapp/lib/python2.7/site-packages/rlp/codec.py", line 124, in consume_length_prefix
    b0 = safe_ord(rlp[start])
TypeError: ord() expected string of length 1, but int found
<Greenlet at 0x7fe5b969b190: <bound method Peer._run_decoded_packets of <Peer('127.0.0.1', 41306) pyethapp/v1.4.0>>> failed with TypeError

[coveralls]

Coverage decreased (-0.09%) to 88.021% when pulling e1c8081 on janx:fix-payload-type-in-multiplexer into 08b4862 on ethereum:develop.

[coveralls]

Coverage decreased (-0.09%) to 88.021% when pulling e1c8081 on janx:fix-payload-type-in-multiplexer into 08b4862 on ethereum:develop.