python: Encode query string as a utf-8 byte array

If we pass in an unicode instance as query string, we would probably get
weird behavior (and indeed do so, see mail
id:"20110707113700.GA16347@megatron"). If a unicode instance is passed
in, make sure we encode it properly to an utf-8 encoded byte string.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>