Expiration Header

[ishields]

I was curious whether it would make sense to add a gem configuration of specifying an expiration header? In a lot of use cases these urls do not change so specifying an expiration at a header level would improve performance. If you use a CDN like cloudflare these headers can be used to cache at the CDN level too so it doesn't ever hit the app server.

Rails expires_in header documentation

[fschwahn]

Sounds sensible, can you open a PR with your proposed changes?