Bump rack from 1.6.11 to 1.6.12

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

@@ -170,7 +170,7 @@ GEM
       nio4r (~> 2.0)
     quiet_assets (1.1.0)
       railties (>= 3.1, < 5.0)
-    rack (1.6.11)
+    rack (1.6.12)
     rack-mini-profiler (1.0.2)
       rack (>= 1.2.0)
     rack-test (0.6.3)