🐛 Fix bug that prevents loading assets with webpacker-dev-server

Issue reactjs#739

In certain circumstances `webpacker-dev-server` includes the full path
(protocol and host with port) in the paths generated for the
packs/manifest.json file. This addition removes the protocol, host, and
port from the asset path when present allowing assets to be correctly
loaded.

lib/react/server_rendering/webpacker_manifest_container.rb

@@ -42,6 +42,8 @@ def find_asset(logical_path)
           asset_path = Webpacker.manifest.lookup(logical_path).to_s
           if Webpacker.dev_server.running?
             ds = Webpacker.dev_server
+            # Remove the protocol and host from the asset path. Sometimes webpacker includes this, sometimes it does not
+            asset_path.slice!("#{ds.protocol}://#{ds.host_with_port}")
             dev_server_asset = open("#{ds.protocol}://#{ds.host_with_port}#{asset_path}").read
             dev_server_asset.sub!(CLIENT_REQUIRE, '//\0')
             dev_server_asset