fix non-ascii chars in command arguments

to trigger the error, try "/otr smp respond gemüse"

this reverts part of 95460bdec843b9f63d2577b7093cc06e592f44a6 to avoid a UnicodeEncodeError in shlex:

~~~~
Traceback (most recent call last):
  File "~/.weechat/python/autoload/otr.py", line 1123, in command_cb
    arg_parts = shlex.split(PYVER.to_unicode(args))
  File "/usr/lib/python2.7/shlex.py", line 275, in split
    lex = shlex(s, posix=posix)
  File "/usr/lib/python2.7/shlex.py", line 25, in __init__
    instream = StringIO(instream)
UnicodeEncodeError: 'ascii' codec can't encode character u'\xfc' in position 14: ordinal not in range(128)
~~~~