Fix exception: TypeError: can't concat bytes to str

[ichramm]

Found when studying issue #24

Traceback (most recent call last):
  File "/usr/lib/python3.6/socketserver.py", line 639, in process_request_thread
    self.finish_request(request, client_address)
  File "/usr/lib/python3.6/socketserver.py", line 361, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/usr/lib/python3.6/socketserver.py", line 696, in __init__
    self.handle()
  File "./pyicap.py", line 443, in handle
    self.handle_one_request()
  File "./pyicap.py", line 500, in handle_one_request
    self.send_error(e.code, e.message[0])
  File "./pyicap.py", line 525, in send_error
    self.set_icap_response(code, message=message)
  File "./pyicap.py", line 267, in set_icap_response
    (message if message else self._responses[code][0])
TypeError: can't concat bytes to str
----------------------------------------

[netom]

Thank you for the fix.