[WIP] Allow override of request cache headers

This PR corresponds to feature #99.
Any comments welcome (I know, tests are missing..)
can you add an ->info() here to say that this is to tell the *default* behaviour?

makes me wonder if we want to move that into its own group called defaults, so that it would be `cache_control.defaults.overwrite`. maybe more clean, especially if we add more defaults for stuff later on.