Deprecation Warning http-errors - createError
`http-errors` has a deprecation warning for calls to `createError()` where the first argument is non-numeric.  This happens when `throw()` is called in [`context.js [96]`](https://github.com/koajs/koa/blob/master/lib/context.js#L75-L97), which allows for non-numeric first parameters.  The documentation for `throw()` should be updated to indicate that the first parameter should be the HTTP response code.

The deprecation warning can be found in `http-errors`, [`index.js [68]`](https://github.com/jshttp/http-errors/blob/master/index.js#L68)