Add RSpec 4 compatibility (#17652)

* Remove outdated option

04078317ba

* Use the non-globally exposed RSpec syntax

https://github.com/rspec/rspec-core/pull/2803

* Use the non-globally exposed RSpec syntax, cont

https://github.com/rspec/rspec-core/pull/2803

* Comply to strict predicate matchers

See:
 - https://github.com/rspec/rspec-expectations/pull/1195
 - https://github.com/rspec/rspec-expectations/pull/1196
 - https://github.com/rspec/rspec-expectations/pull/1277

spec/lib/middleware/request_tracker_spec.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-describe Middleware::RequestTracker do
+RSpec.describe Middleware::RequestTracker do
   def env(opts = {})
     create_request_env.merge(
       "HTTP_HOST" => "http://test.com",