fix: Add Accept header to requests in NetworkLoader

This allows development servers to serve the correct file type.

For more information see ElMassimo/vite_ruby#93 (comment)

lib/premailer/rails/css_loaders/network_loader.rb

@@ -6,7 +6,7 @@ module NetworkLoader
 
         def load(url)
           uri = uri_for_url(url)
-          Net::HTTP.get(uri) if uri
+          Net::HTTP.get(uri, { 'Accept' => 'text/css' }) if uri
         end
 
         def uri_for_url(url)