Content length response header incorrectly parsed
When requesting the length using `ctx.length`, Koa parses the content length response header using the double tilde operator: https://github.com/koajs/koa/blob/master/lib/response.js#L214 However, this operation also performs a 32-bit conversion, resulting in incorrect and weird results when serving large files. Isn't it therefore better to use the `Math.trunc()` function instead to parse the content length response header?