Bump rack from 1.6.11 to 1.6.12 (sass#422)

Bumps [rack](https://github.com/rack/rack) from 1.6.11 to 1.6.12.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.6.11...1.6.12)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -130,7 +130,7 @@ GEM
       activesupport (>= 3.1)
     parallel (1.18.0)
     puma (3.12.2)
-    rack (1.6.11)
+    rack (1.6.12)
     rack-conditional (0.3.1)
       rack (>= 1.6.11)
     rack-contrib (1.8.0)