Calculate prefetched value

Calculate the prefetched value as opposed to a prefetchType

`spfPrefetch` contains the boolean value of whether the request was a prefetch or not.

`spfPrefetch && !spfCached` implies the request was a promote
`spfPrefetch && spfCached` implies a prefetch cache hit
Make this `!!cached && cached.type == 'prefetch'` so that the value will always be `true` or `false`, even if `getCacheObject_` returns `null`