utils: Fix bytearray conversion

Parameter buf is instance of bytearray, but Ryu tries to convert it as string,
and outputs error messages as a result.
This patch fixes this problem.

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
utils: Add unit test for ryu.utils

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>