rapi.testutils: handle bytes
rapi.testutils: handle bytes

Make sure we pass bytes to b64encode and decode the response body as
UTF-8.

Signed-off-by: Apollon Oikonomopoulos <ed81ed0a45ccd5c8869bda52e38353a6299ee39e@dmesg.gr>

ganeti/ganeti,ganeti/ganeti,ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,mbakke/ganeti