Make sure we keep the stable-id and origin-id nodes when required

See https://xmpp.org/extensions/xep-0359.html

tests/end_to_end/scenarios/stable_id.py

@@ -0,0 +1,26 @@
+from scenarios import *
+
+import scenarios.simple_channel_join
+
+# see https://xmpp.org/extensions/xep-0359.html
+
+scenario = (
+    scenarios.simple_channel_join.scenario,
+
+    send_stanza("""<message id='first_id' from='{jid_one}/{resource_one}' to='#foo%{irc_server_one}' type='groupchat'>
+      <origin-id xmlns='urn:xmpp:sid:0' id='client-origin-id'/>
+      <stanza-id xmlns='urn:xmpp:sid:0' id='client-stanza-id'/>
+      <body>coucou</body></message>"""),
+
+    # Entities, which are routing stanzas, SHOULD NOT strip any elements
+    # qualified by the 'urn:xmpp:sid:0' namespace from message stanzas
+    # unless the preceding rule applied to those elements.
+    expect_stanza("/message/stable_id:origin-id[@id='client-origin-id']",
+    # Stanza ID generating entities, which encounter a <stanza-id/>
+    # element where the 'by' attribute matches the 'by' attribute they
+    # would otherwise set, MUST delete that element even if they are not
+    # adding their own stanza ID.
+                  "/message/stable_id:stanza-id[@id][@by='#foo%{irc_server_one}']",
+                  "!/message/stable_id:stanza-id[@id='client-stanza-id']",
+    ),
+)