python3: Fix byte/string handling in PING request
python3: Fix byte/string handling in PING request

The ping request fails under Python 3 due to incorrect byte/string handling.

Test by sending a HTTP request to locally running frontend and backend with
Python 2 and 3. Also connect to pagekite.net service with Python 2 and 3.

pagekite/PyPagekite,pagekite/PyPagekite,pagekite/PyPagekite