Support custom `Vary` header

[nicksrandall]

I'm admittedly new to Rust so correct me where I'm wrong but it appears to me that a user is only able to construct a Vary: * header and is not able to construct a "custom" Vary header like Vary: accept-encoding.

If I'm correct, I'd be happy to submit a PR that enables creation of custom Vary headers though I'd probably need some direction from maintainers on what the API should look like.