use correct cache control headers

[fredrikj31]

It would be nice if the plugin follows by default the Cache Control headers directive. So clients has the correct hints about the caching, and as well implementers has this out of the box.

[gdgunnars]

Could we consider this as a config setting enabling/disabling ?

[fredrikj31]

We could do that. I have just used x-cache: (hit/miss), for debugging purposes. But if we want browsers to cache responses, then we should probably also return the correct headers, to browsers respect them.

[gdgunnars]

Defaulting to true is fine