utils: Fix bytearray conversion

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

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
test_utils: Add unit test for ryu.utils

Signed-off-by: IWASE Yusuke <iwase.yusuke0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>