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)

spec/integration/rate_limiting_spec.rb

@@ -24,7 +24,7 @@ describe 'rate limiter integration' do
     }
 
     expect(response.status).to eq(429)
-    expect(response.headers['Retry-After']).to be > 29
+    expect(response.headers['Retry-After'].to_i).to be > 29
   end
 
   it "will not rate limit when all is good" do
@@ -76,7 +76,7 @@ describe 'rate limiter integration' do
 
     data = response.parsed_body
 
-    expect(response.headers['Retry-After']).to eq(60)
+    expect(response.headers["Retry-After"]).to eq("60")
     expect(data["extras"]["wait_seconds"]).to eq(60)
   end
 end