Update rack-test requirement from ~> 1.1 to ~> 2.0

Updates the requirements on [rack-test](https://github.com/rack/rack-test) to permit the latest version.
- [Release notes](https://github.com/rack/rack-test/releases)
- [Changelog](https://github.com/rack/rack-test/blob/main/History.md)
- [Commits](rack/rack-test@v1.1.0...v2.0.0)

---
updated-dependencies:
- dependency-name: rack-test
  dependency-type: direct:development
...

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

split.gemspec

@@ -36,7 +36,7 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency "bundler",     ">= 1.17"
   s.add_development_dependency "simplecov",   "~> 0.15"
-  s.add_development_dependency "rack-test",   "~> 1.1"
+  s.add_development_dependency "rack-test",   "~> 2.0"
   s.add_development_dependency "rake",        "~> 13"
   s.add_development_dependency "rspec",       "~> 3.7"
   s.add_development_dependency "pry",         "~> 0.10"