fix(middleware): don't use headers.append(), use headers.set()

[yusukebe]

Some middleware are using response.headers.append() when setting header values. With this method, it's possible to set duplicate values if the middleware is called twice.

app.use(
  '/api/*',
  cors({
    origin: 'http://example.com',
  })
)
app.use(
  '/api/*',
  cors({
    origin: 'http://example.com',
  })
)

In this PR, response.headers.set() is used to fix this issue.