Always use the provided port for protocol relative urls

There may be situations where you need to tunnel SSL connections over
port 80 so we shouldn't remove it if it has been explicitly provided.

actionpack/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Always use the provided port if the protocol is relative.
+
+    Fixes #15043.
+
+    *Guilherme Cavalcanti*, *Andrew White*
+
 *   Moved `params[request_forgery_protection_token]` into its own method
     and improved tests.
 

actionpack/lib/action_dispatch/http/url.rb

@@ -135,6 +135,8 @@ module ActionDispatch
           return nil if options[:port].nil? || options[:port] == false
 
           case options[:protocol]
+          when "//"
+            options[:port]
           when "https://"
             options[:port].to_i == 443 ? nil : options[:port]
           else

actionpack/test/dispatch/url_generation_test.rb

@@ -64,18 +64,30 @@ module TestUrlGeneration
 
     test "port is extracted from the host" do
       assert_equal "http://www.example.com:8080/foo", foo_url(host: "www.example.com:8080", protocol: "http://")
+      assert_equal "//www.example.com:8080/foo", foo_url(host: "www.example.com:8080", protocol: "//")
+      assert_equal "//www.example.com:80/foo", foo_url(host: "www.example.com:80", protocol: "//")
+    end
+
+    test "port option is used" do
+      assert_equal "http://www.example.com:8080/foo", foo_url(host: "www.example.com", protocol: "http://", port: 8080)
+      assert_equal "//www.example.com:8080/foo", foo_url(host: "www.example.com", protocol: "//", port: 8080)
+      assert_equal "//www.example.com:80/foo", foo_url(host: "www.example.com", protocol: "//", port: 80)
     end
 
     test "port option overrides the host" do
       assert_equal "http://www.example.com:8080/foo", foo_url(host: "www.example.com:8443", protocol: "http://", port: 8080)
+      assert_equal "//www.example.com:8080/foo", foo_url(host: "www.example.com:8443", protocol: "//", port: 8080)
+      assert_equal "//www.example.com:80/foo", foo_url(host: "www.example.com:443", protocol: "//", port: 80)
     end
 
     test "port option disables the host when set to nil" do
       assert_equal "http://www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "http://", port: nil)
+      assert_equal "//www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "//", port: nil)
     end
 
     test "port option disables the host when set to false" do
       assert_equal "http://www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "http://", port: false)
+      assert_equal "//www.example.com/foo", foo_url(host: "www.example.com:8443", protocol: "//", port: false)
     end
 
     test "keep subdomain when key is true" do