1. when forming instance parameters xml dict on client side, we cannot
   use str() to convert parameter_value to string - because it breaks e.g.
   this way:

   In [1]: s = u'Привет Мир!'
   In [2]: str(s)
   ---------------------------------------------------------------------------
   UnicodeEncodeError                        Traceback (most recent call

   UnicodeEncodeError: 'ascii' codec can't encode characters in position
   0-5: ordinal not in range(128)

   Fix is: convert arguments to unicode strings.

2. when decoding xml on server side it was erroneously assumed to be
   always unicode, and this way xml.encode('utf-8') fails if xml is str
   but with non-ascii characters:

   In [3]: r = 'hello world'

   In [4]: r.encode('utf-8')
   Out[4]: 'hello world'

   In [5]: t = s.encode('utf-8')

   In [6]: t
   Out[6]: '\xd0\x9f\xd1\x80\xd0\xb8\xd0\xb2\xd0\xb5\xd1\x82 \xd0\x9c\xd0\xb8\xd1\x80!'

   In [7]: t.encode('utf-8')
   ---------------------------------------------------------------------------
   UnicodeDecodeError                        Traceback (most recent call last)

   UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128)

   Fix is: don't assume xml string is unicode and decode it only, if it
           is unicode, and leave as it is if it is just str.