PERF: include content-length header for CDN

Attempt to force NGINX to include content length when doing X-SendFile
This does not seem to be required when bypassing NGINX.

Without this header some CDNs may have issues caching

spec/requests/theme_javascripts_controller_spec.rb

@@ -40,6 +40,7 @@ describe ThemeJavascriptsController do
       get "/theme-javascripts/#{javascript_cache.digest}.js"
       expect(response.status).to eq(200)
       expect(response.body).to eq(javascript_cache.content)
+      expect(response.headers['Content-Length']).to eq(javascript_cache.content.bytesize.to_s)
 
       javascript_cache.destroy!