String instance is broken with non-ASCII data

Although the package works with String, and indeed encourages its use by having that be the default type in some of the API helper functions, the handling of non-ASCII data is completely broken. We should be doing encoding properly to be consistent with the Content-Type header, both when sending and receiving.

This has been reported by a couple of users so far and is clearly pretty nasty, but I don't think it's trivial to fix because presumably the receiving and sending sides can use different encodings and when sending requests we ought to be optionally giving the user control of the encoding.

The alternative of removing the String instances completely is likely to be very disruptive so I don't think it's a reasonable option.

まず、データの送信と受信を行う String API ヘルパーの経路を追跡し、次に各経路が Content-Type ヘッダーと文字エンコーディングをどのように扱うかを調べます。両側で使用されているエンコーディングを比較し、呼び出し元がそれを制御できる箇所を特定します。String インスタンスを削除せずに、送受信時の非 ASCII String データが一貫して処理されれば完了です。

索引モデルが issue の本文から書いたものです。