DEV: Retry-after header values should be strings (#12475)

Fixes `Rack::Lint::LintError: a header value must be a String, but the value of 'Retry-After' is a Integer`. (see: 14a236b4f0/lib/rack/lint.rb (L676))

I found it when I got flooded by those warning a while back in a test-related accident 😉 (ember CLI tests were hitting a local rails server at a fast rate)

lib/middleware/request_tracker.rb

@@ -166,7 +166,7 @@ class Middleware::RequestTracker
     if available_in = rate_limit(request)
       return [
         429,
-        { "Retry-After" => available_in },
+        { "Retry-After" => available_in.to_s },
         ["Slow down, too many requests from this IP address"]
       ]
     end