Add a linter for rack.early_hints

[casperisfine]

Fix: #1692

This is kind of a first stab, I'm not familiar enough with Rack::Lint to understand exactly what's needed. Please let me know if there are specifications I missed, or if the main description should be improved.

cc @jeremyevans

[ioquatix]

I'm okay with this, but I think we should defer to the response headers rather than re-specifying it to avoid duplicating the specifications.

[jeremyevans]

With the changes to SPEC since this was originally submitted, I think this will require significant changes before merging. rack.early_hints should be called with an argument that would be valid as response headers. The Lint implementation can call check_headers with the value.

@casperisfine Unless you want to work on this, I can see if I can update it tomorrow.

[casperisfine]

I don't absolutely want to work on it, feel free to take over if you wish, but otherwise I can probably do that in a day or two.

[casperisfine]

Sorry for the delay, I rebased and replaced the content of check_early_hints by check_headers.

It's been a while, so I hope the result is decent. Again feel free to directly take over if you'd rather quickly improve it rather than go through review rounds.

[jeremyevans]

@casperisfine Sorry for not being more explicit about it, but I already submitted a follow-up PR that includes specs: #1831

[casperisfine]

Ah that great!