Lowercase headers

[arashnd]

All PRs:

• Has tests
• Documentation updated

Adding a new header

Generally, adding a new header is always OK.

• Is the header supported by any user agent? If so, which?
• What does it do?
• What are the valid values for the header?
• Where does the specification live?

Adding a new CSP directive

• Is the directive supported by any user agent? If so, which?
• What does it do?
• What are the valid values for the directive?

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Rack 3 Response header keys can no longer include uppercase characters.
https://github.com/rack/rack/blob/main/UPGRADE-GUIDE.md#rack-3-upgrade-guide

Rack Lint error out when using secure_headers because its adding headers with uppercase

Rack::Lint::LintError: uppercase character in header name: X-Frame-Options (Rack::Lint::LintError)

[obrie]

Thanks for working on this! It'd be great to have this merged and released to unblock local development with Rack 3.x.

[rzhade3]

Thanks for this PR! This is a great change. Couple changes suggested below after which we can merge this PR in:

[arashnd]

thanks! @rzhade3