(MAINT) Adjust tests after newest json release

Following the latest release of the json gem, the tests
have broken as a result of a change in behaviour. This commit
aims to adjust the tests to work with the new version.

Gemfile

@@ -16,9 +16,10 @@ group :development do
   gem 'yard', '~> 0.9.28',                :require => false
   gem "rubocop", '~> 1.64.0',             :require => false
   gem "rubocop-performance", '~> 1.16',   :require => false
-  gem "rubocop-rspec", '~> 3.0',         :require => false
+  gem "rubocop-rspec", '~> 3.0',          :require => false
   gem 'simplecov',                        :require => false
   gem 'simplecov-console',                :require => false
+  gem 'json', "< 2.8.0",                  :require => false
 
   if ENV['PUPPET_GEM_VERSION']
     gem 'puppet', ENV['PUPPET_GEM_VERSION'], :require => false

spec/languageserver/unit/puppet-languageserver/sidecar_protocol_spec.rb

@@ -9,7 +9,7 @@
       end
     end
 
-    it 'should serialize to a string' do
+    it 'should serialize to an string' do
       serial = subject.to_json
 
       expect(serial).to be_a(String)