How to fix this notice from google pagespeed?
The following resources are missing a cache validator. Resources that
do not specify a cache validator cannot be refreshed efficiently.
Specify a Last-Modified or ETag header to enable cache validation for
the following resources:
https://MY_DOMAIN.com/piwik.php?action_name=REMOVED&idsite=2&rec=1&r=447885&h=4&m=18&s=26&url=https%3A%2F%MY_DOMAIN.com%2F&_id=42720adf0bd62774&_idts=1476703106&_idvc=1&_idn=0&_refts=0&_viewts=1476703106&send_image=1&pdf=0&qt=0&realp=0&wma=0&dir=0&fla=1&java=0&gears=0&ag=0&cookie=1&res=1024x861>_ms=173