Use lower case constants for headers (fixes Rack > 2 applications)

[davidalejandroaguilar]

On Rack 3 https://github.com/rack/rack/issues/1592, it was decided to lowercase all response headers. Thus, all logic in the codebase that checks for headers coming from Rack > 2 applications is broken.

This PR:

• Uses lower case constants for headers.
• It does so by checking Rack.release > "2" so that we keep compatibility with Rack 2.x and 1.6.x.
  • This introduces if statements that look a bit ugly, but that was the pattern I found was being used within the codebase.

Note

I'm yet to write some specs for this but wanted to get this draft out for potential discussion.

[tomasc]

see #682

[janko]

Closing in favor of https://github.com/shrinerb/shrine/pull/682.