Implement H2 Early Hints for Rails

When puma/puma#1403 is merged Puma will support the Early Hints status
code for sending assets before a request has finished.

While the Early Hints spec is still in draft, this PR prepares Rails to
allowing this status code.

If the proxy server supports Early Hints, it will send H2 pushes to the
client.

This PR adds a method for setting Early Hints Link headers via Rails,
and also automatically sends Early Hints if supported from the
`stylesheet_link_tag` and the `javascript_include_tag`.

Once puma supports Early Hints the `--early-hints` argument can be
passed to the server to enable this or set in the puma config with
`early_hints(true)`. Note that for Early Hints to work
in the browser the requirements are 1) a proxy that can handle H2,
and 2) HTTPS.

To start the server with Early Hints enabled pass `--early-hints` to
`rails s`.

This has been verified to work with h2o, Puma, and Rails with Chrome.

The commit adds a new option to the rails server to enable early hints
for Puma.

Early Hints spec:
https://tools.ietf.org/html/draft-ietf-httpbis-early-hints-04

[Eileen M. Uchitelle, Aaron Patterson]

2 files changed, 8 insertions, 1 deletions

diff --git a/actionview/test/template/asset_tag_helper_test.rb b/actionview/test/template/asset_tag_helper_test.rb
index 182d8f89f7..7475f5cc3c 100644
--- a/actionview/test/template/asset_tag_helper_test.rb
+++ b/actionview/test/template/asset_tag_helper_test.rb
@@ -19,6 +19,7 @@ class AssetTagHelperTest < ActionView::TestCase
       def ssl?() false end
       def host_with_port() "localhost" end
       def base_url() "http://www.example.com" end
+      def send_early_hints(links) end
     end.new
 
     @controller.request = @request
@@ -653,7 +654,9 @@ class AssetTagHelperNonVhostTest < ActionView::TestCase
     @controller = BasicController.new
     @controller.config.relative_url_root = "/collaboration/hieraki"
 
-    @request = Struct.new(:protocol, :base_url).new("gopher://", "gopher://www.example.com")
+    @request = Struct.new(:protocol, :base_url) do
+      def send_early_hints(links); end
+    end.new("gopher://", "gopher://www.example.com")
     @controller.request = @request
   end
 
diff --git a/actionview/test/template/javascript_helper_test.rb b/actionview/test/template/javascript_helper_test.rb
index 4478c9f4ab..a72bc6c2fe 100644
--- a/actionview/test/template/javascript_helper_test.rb
+++ b/actionview/test/template/javascript_helper_test.rb
@@ -6,11 +6,15 @@ class JavaScriptHelperTest < ActionView::TestCase
   tests ActionView::Helpers::JavaScriptHelper
 
   attr_accessor :output_buffer
+  attr_reader :request
 
   setup do
     @old_escape_html_entities_in_json = ActiveSupport.escape_html_entities_in_json
     ActiveSupport.escape_html_entities_in_json = true
     @template = self
+    @request = Class.new do
+      def send_early_hints(links) end
+    end.new
   end
 
   def teardown