feat(middleware): support overwriting response after next()

[yusukebe]

This PR enables completely overwriting the response from middleware after next(). This matter is mentioned at #960 .

We can do it by putting undefined:

app.use('*', async (c, next) => {
  await next()
  c.res = undefined
  c.res = new Response('New Response')
})

Good PR, as this is done with very few codes.

Fix #960