ctx.state not being passed down to middlware
Hello, I've originally encountered a problem with JQuery and koa-helmet's CSP, but found a solution by generating nonce for JQuery. Now my problem is by following the [helmet ](https://helmetjs.github.io/docs/csp/) documentation on how to generate nonces, I can't seem to replicate it in KOA2, my app is throwing an error saying ctx.state is undefined.

The code:
`.use(helmet.contentSecurityPolicy({
  directives: {
    defaultSrc: ["'self'"],
    scriptSrc: ["'self'", (ctx) => { return `'nonce-${ctx.state.nonce}'` }],
    styleSrc: ["'self'", (ctx) => { return `'nonce-${ctx.state.nonce}'` }]
  }
}))`