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]

1 files changed, 12 insertions, 0 deletions

diff --git a/railties/test/commands/server_test.rb b/railties/test/commands/server_test.rb
index 556c2289e7..a6201e4f04 100644
--- a/railties/test/commands/server_test.rb
+++ b/railties/test/commands/server_test.rb
@@ -81,6 +81,18 @@ class Rails::ServerTest < ActiveSupport::TestCase
     assert_equal false, options[:caching]
   end
 
+  def test_early_hints_with_option
+    args = ["--early-hints"]
+    options = parse_arguments(args)
+    assert_equal true, options[:early_hints]
+  end
+
+  def test_early_hints_is_nil_by_default
+    args = []
+    options = parse_arguments(args)
+    assert_nil options[:early_hints]
+  end
+
   def test_log_stdout
     with_rack_env nil do
       with_rails_env nil do