Pages section is still required if key function is provided

[ludwis]

key function comment doc says:

// custom function to return cache key, when used previous
// properties (useHostPrefix, pages) are ignored.

That is not true, if pages is not set with at least one element, cache is not initialiazed at all due the if statement: https://github.com/arash16/nuxt-ssr-cache/blob/master/src/middleware.js#L34