Don't let an http_proxy environment variable bypass the pinned address in link previews

Opengraph::Fetch connects to the IP it checked, not to a fresh lookup of
the host. With http_proxy set, Net::HTTP would instead send the hostname
to the proxy, which looks it up again, so the connection could end up
somewhere other than the checked address. Pass nil as the proxy address,
as web push delivery already does.

(cherry picked from commit a7e9ea53dd)

test/models/opengraph/fetch_test.rb

@@ -74,6 +74,23 @@ class Opengraph::FetchTest < ActiveSupport::TestCase
     end
   end
 
+  test "#fetch_document connects to the resolved IP even when a proxy is configured" do
+    url = URI.parse("http://www.example.com/")
+    saved = ENV.slice("http_proxy", "HTTP_PROXY")
+    %w[ http_proxy HTTP_PROXY ].each { |k| ENV[k] = "http://proxy.internal:3128" }
+
+    WebMock.disable_net_connect! allow: [ url.host ]
+    TCPSocket.expects(:open).with { |*args, **| args.first == "proxy.internal" }.never
+    TCPSocket.expects(:open).with { |*args, **| args.first == "1.2.3.4" && args[1] == 80 }.throws(:not_proxied)
+
+    assert_throws :not_proxied do
+      @fetch.fetch_document(url, ip: "1.2.3.4")
+    end
+  ensure
+    %w[ http_proxy HTTP_PROXY ].each { |k| ENV.delete(k) }
+    saved.each { |k, v| ENV[k] = v }
+  end
+
   test "#fetch_document is empty following redirects that never finish" do
     WebMock.stub_request(:get, "https://www.example.com/")
       .to_return(status: 302, headers: { location: "https://www.example.com/" })