going for an explicit list of headers. Response allows to set etag and last_modified in the `setCache` method. i propose to change the semantics of last_modified (only set if not already set, as we probably have a less good idea than a controler had) and move it out to its own header (and call setLastModified directly). and i simply drop etag here because its utterly pointless. people need their own code to figure out what the etag of their response can be, no global configuration can tell that.