`CachePolicy` object that computes properties of a HTTP response, such as whether it's fresh or stale, and how long it can be cached for. Based on RFC 7234.
## Usage
```js
const cache = new CachePolicy(request, response, options);
// Age counts from the time response has been created
const secondsFresh = cache.maxAge();
const secondsOld = cache.age();
// Current state
const currentState = cache.isFresh();
```
Cacheability of response depends on how it was requested, so both request and response are required. Both are objects with `headers` property that is an object with lowercased header names as keys, e.g.
```js
const request = {
method: 'GET',
headers: {
'accept': '*/*',
},
};
const response = {
status: 200,
headers: {
'cache-control': 'public, max-age=7234',
},
};
const options = {
shared: true,
};
```
If `options.shared` is true (default), then response is evaluated from perspective of a shared cache (i.e. `private` is not cacheable and `s-maxage` is respected). If `options.shared` is false, then response is evaluated from perspective of a single-user cache (i.e. `private` is cacheable and `s-maxage` is ignored).
