Cleanup usages of `JSON.parse` in specs

...controllers/admin/application_settings_controller_spec.rb

spec/controllers/application_controller_spec.rb

spec/controllers/boards/issues_controller_spec.rb

spec/controllers/boards/lists_controller_spec.rb

spec/controllers/groups/boards_controller_spec.rb

spec/controllers/groups/milestones_controller_spec.rb

spec/controllers/health_check_controller_spec.rb

spec/controllers/health_controller_spec.rb

spec/controllers/metrics_controller_spec.rb

spec/controllers/projects/blob_controller_spec.rb

spec/controllers/projects/boards_controller_spec.rb

spec/controllers/projects/branches_controller_spec.rb

spec/controllers/projects/commit_controller_spec.rb

spec/controllers/projects/compare_controller_spec.rb

spec/controllers/projects/deploy_keys_controller_spec.rb

spec/controllers/projects/discussions_controller_spec.rb

spec/controllers/projects/find_file_controller_spec.rb

spec/controllers/projects/issues_controller_spec.rb

...trollers/projects/merge_requests/diffs_controller_spec.rb

spec/controllers/projects/merge_requests_controller_spec.rb