Add RFC-8246 "immutable" attribute to CacheControl

[making]

This commit adds the "immutable" Cache-Control directives in CacheControl.

[bclozel]

Thanks for the PR @making !
The CacheControl API tries to guide developers towards HTTP best practices for "Cache-Control" header values, with static factory methods and instance methods.

In this case, the popular use case uses both a max-age and the immutable directive, like so: "Cache-Control: max-age=31536000, immutable". The goal here is to instruct the browser that the response should not be revalidated as long as it's not stale, giving a long "max-age" value.

We could have enforced this by requiring a "max-age" value if the freshness directive is used. This is not technically enforced by the RFC. Without such a value, browsers are likely to fall back to heuristics caching, effectively caching the resource for (current date - last modified date) / 10.

At the time of this comment, numerous browsers aren't supporting this directive: Edge, Chrome, Android browsers, Opera. Because of that, most implementations should use a max-age value in all cases for compatibility.

We won't enforce the need for max-age, but we should probably guide developers there with the Javadoc.