added draft of push promise plugin

lib/httpx/plugins/push_promise.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+module HTTPX
+  module Plugins
+    module PushPromise
+      PUSH_OPTIONS = { http2_settings: { settings_enable_push: 1 } }
+
+      module InstanceMethods
+        def initialize(opts = {})
+          super(PUSH_OPTIONS.merge(opts))
+        end
+      end
+    end
+    register_plugin(:push_promise, PushPromise)
+  end
+end 

test/http2_test.rb

@@ -16,6 +16,7 @@ class HTTP2Test < HTTPTest
   include Plugins::FollowRedirects
   include Plugins::Cookies
   include Plugins::Compression
+  include Plugins::PushPromise
 
   private
 

test/support/requests/plugins/push_promise.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+module Requests
+  module Plugins
+    module PushPromise
+      def test_plugin_push_promise_get_all
+        client = HTTPX.plugin(:push_promise)
+        html, css = client.get(push_html_uri, push_css_uri)
+        verify_status(html.status, 200)
+        verify_status(css.status, 200)
+      end
+
+
+      private
+
+      def push_origin
+        "https://nghttp2.org"
+      end
+
+      def push_html_uri
+        "#{push_origin}/"
+      end
+      
+      def push_css_uri
+        "#{push_origin}/stylesheets/screen.css" 
+      end
+    end
+  end
+end
+