Given that /ipfs/ resources never change, we can easily benefit from this
new cache control extension.

Support for this is about to land in Firefox 49:

https://bugzilla.mozilla.org/show_bug.cgi?id=1267474

For more details, there is a good blogpost about this here:

https://bitsup.blogspot.de/2016/05/cache-control-immutable.html

This header is ignored by clients that don't support it.

License: MIT
Signed-off-by: kpcyrd <git@rxv.cc>