Response headers will be ignored/removed while using ctx.throw.
```js
ctx.set( 'Access-Control-Allow-Origin', '*' );
ctx.throw( 404 );
```
By using the code above, the header of `Access-Control-Allow-Origin` will not be sent with responding. Even though I can use `res.statusCode` instead, but I think the `ctx.throw` method should not remove response headers.

Koa@2.4.1
Node@8.9.3