Fix header casing compatibility with Rails 7

[skipkayhil]

Ref rails/rails#48874
Fixes rails/rails#47096 for Rails < 7.1

To support Rack 3, response headers in Sprockets::Server were all downcased. However, this has led to issues with Rack 2 applications (ex. Rails 7) since they still expect mixed case (ex. Content-Type) headers.

To ensure compatibility with both Rack 2 and Rack 3 applications, this commit makes the casing of the headers conditional on the Rack version. Rack itself provides constants to do this easily for most of the headers used (Content-Type, Content-Length, Cache-Control, and ETag) and the rest are added as constants under Rack::Server.

As an alternative to this, the responses could instead be wrapped using Rack::Headers (and Rack::Utils::HeaderHash in Rack 2), but making the header casing conditional seems better to me because it is relatively easier to implement and there will be less churn if/when Rack 2 support is eventually removed.

[skipkayhil]

Tests should be fixed by #791

[rafaelfranca]

Wait, what?

Doesn't rack >= 2.2.4 don't care about the casing of the headers? #758 (comment)

[skipkayhil]

Doesn't rack >= 2.2.4 don't care about the casing of the headers?

For any library that aims to support Rack 2 and Rack 3, the casing of headers matters unless those libraries return Rack::Utils::HeaderHash/Rack::Headers in all responses and all middleware that read/write headers use Rack::Utils::HeadersHash/Rack::Headers as well.

So Rails returns HeadersHash because it uses ActionDispatch::Response, but these responses are plain hashes. This means a Rack 2 middleware immediately above Sprockets::Server that tries to read Content-Type will not work correctly since these responses are hashes that only contain content-type.

---

So as an alternative to this PR, we could apply Headers to these responses, something like this:

def bad_request_response(env)
  if head_request?(env)
    [ 400, Headers[{ "content-type" => "text/plain", Rack::CONTENT_LENGTH => "0" }], [] ]
  else
    [ 400, Headers[{ "content-type" => "text/plain", Rack::CONTENT_LENGTH => "11" }], [ "Bad Request" ] ]
  end
end

where Headers looks like: https://github.com/rails/rails/blob/main/actionpack/lib/action_dispatch/http/response.rb#L37-L45

---

I wrote some thoughts on using Headers vs conditional casing here: rails/rails#48874 (comment) and rails/rails#48874 (comment)

[rafaelfranca]

Suggested change

	X_CASCADE = "X-Cascade"
	VARY = "Vary"
	X_CASCADE = "X-Cascade" # :nodoc:
	VARY = "Vary" # :nodoc:

I don't want to maintain constants for strings just because Rack 3 is incompatible with Rack 2