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
Please use the standard formatting:
```js
var cacheValue = {
  response: response,
  type: type
}
```