POST requests should be in Shift_JIS encoding?

While I was testing some convenience store payments, sometimes I was getting error

M01010013 / 氏名に不正な文字が含まれています。
and
M01011013 / フリガナに不正な文字が含まれています。

So, I did a little more digging and found that POST requests should be in Shift_JIS encoding (021_制限事項一覧_1.19.pdf page 23)

I'm not sure if I did it the right way in this pull request, but it seems to be working now.