Cache Validation

| Q | A |
| --- | --- |
| Bug? | no |
| New Feature? | yes |

See php-http/plugins#57, @dbu wrote: 

> the CachePlugin should handle Last-Modified and Etag headers and send If-None-Match resp. If-Modified-Since headers along if the data is still in the cache. this would allow the server to respond with a 304 Not Modified and we could update the cached response and then return that.
