Update rack-test requirement from ~> 0.6 to ~> 1.1

Updates the requirements on [rack-test](https://github.com/rack-test/rack-test) to permit the latest version.
- [Release notes](https://github.com/rack-test/rack-test/releases)
- [Changelog](https://github.com/rack-test/rack-test/blob/master/History.md)
- [Commits](rack/rack-test@v0.6.0...v1.1.0)

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

Author: dependabot-preview[bot]

split.gemspec

@@ -35,7 +35,7 @@ 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',   '~> 0.6'
+  s.add_development_dependency 'rack-test',   '~> 1.1'
   s.add_development_dependency 'rake',        '~> 12'
   s.add_development_dependency 'rspec',       '~> 3.7'
   s.add_development_dependency 'pry',         '~> 0.10'