style(tools): ngx.var.scheme is always lower case

[chronolaw]

Summary

As the nginx's doc and source shows, the var $scheme is always lower case string https,
so the call lower() is unnecessary.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• [Implement ...]

Issue reference

Fix #[issue number]

[chronolaw]

$schema could be rewrite to upper case, so we still need lower()