Enable conditional requests

Enable conditional requests.
Returns a response without body and a 304 status code if the response has not been modified.
This a best practice as its can save a lot of bandwidth.

One can also add a `Last-Modified` date in the response header, `isNotModfied()` will also takes that date into consideration.

BTW, less bandwidth => less energy => less nuclear plants, less pollution => save trees => healthier fauna and flora =>  => better living => happiness
@jasonlewis @philsturgeon ?

Shouldn't this be the reverse? If the option is supplied on a per-group basis then it sets the property on the router.