(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.

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

@@ -9,10 +9,10 @@
       end
     end
 
-    it 'should serialize to a string' do
+    it 'should serialize to an array' do
       serial = subject.to_json
 
-      expect(serial).to be_a(String)
+      expect(serial).to be_a(Array)
     end
 
     it 'should roundtrip to_json to from_json!' do