characters.
 *  New: Accept a charset in `FormBody.Builder`. Previously form bodies were
    always UTF-8.
 *  New: Support the `immutable` cache-control directive.
 *  Fix: Don't crash when an HTTP/2 call is redirected while the connection is
    being shut down.
 *  Fix: Don't drop headers of healthy streams that raced with `GOAWAY` frames.
    This bug would cause HTTP/2 streams to occasional hang when the connection