Link should be separated by comma.

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link.

actionpack/lib/action_dispatch/http/request.rb

@@ -234,7 +234,7 @@ def headers
     #
     # The `send_early_hints` method accepts a hash of links as follows:
     #
-    #     send_early_hints("link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload")
+    #     send_early_hints("link" => "</style.css>; rel=preload; as=style,</script.js>; rel=preload")
     #
     # If you are using `javascript_include_tag` or `stylesheet_link_tag` the Early
     # Hints headers are included by default if supported.

actionpack/test/dispatch/request_test.rb

@@ -1412,8 +1412,8 @@ def setup
   end
 
   test "when early hints is set in the env link headers are sent" do
-    early_hints = @request.send_early_hints("link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload")
-    expected_hints = { "link" => "</style.css>; rel=preload; as=style\n</script.js>; rel=preload" }
+    early_hints = @request.send_early_hints("link" => "</style.css>; rel=preload; as=style,</script.js>; rel=preload")
+    expected_hints = { "link" => "</style.css>; rel=preload; as=style,</script.js>; rel=preload" }
 
     assert_equal expected_hints, early_hints
   end

actionview/lib/action_view/helpers/asset_tag_helper.rb

@@ -657,7 +657,7 @@ def send_preload_links_header(preload_links, max_header_size: MAX_HEADER_SIZE)
           return if response_present && response.sending?
 
           if respond_to?(:request) && request
-            request.send_early_hints("link" => preload_links.join("\n"))
+            request.send_early_hints("link" => preload_links.join(","))
           end
 
           if response_present