Chrome is sending Cache-control:no-cache header

You probably have the Disable cache (while DevTools is open) option selected in the Preferences panel of DevTools.

It's very easy to forget about it! that's what happened to me anyway. Uncheck the option and try again.

I want to add to the accepted answer that refreshing with CMD-SHIFT-R on a Mac or CTRL-SHIFT-R on Windows will also send Cache-Control: no-cache.