Add webrick to support Ruby 3.x (#853)

Author: takahashim

CHANGELOG.md

@@ -6,6 +6,7 @@
 * [#846] (https://github.com/ruby-grape/grape-swagger/pull/846): Fixes oapi rake tasks, allows generating sepcs for different API versions.
 * [#852](https://github.com/ruby-grape/grape-swagger/pull/852): Fix example to work without error - [@takahashim](https://github.com/takahashim)
 * Your contribution here.
+* [#853](https://github.com/ruby-grape/grape-swagger/pull/853): Add webrick gem so that example works in Ruby 3.x - [@takahashim](https://github.com/takahashim)
 
 ### 1.4.2 (October 22, 2021)
 

Gemfile

@@ -28,6 +28,7 @@ group :development, :test do
   gem 'rdoc'
   gem 'rspec', '~> 3.9'
   gem 'rubocop', '~> 1.0', require: false
+  gem 'webrick'
 end
 
 group :test do