Fix sending read receipts in MUCs

- The "by" attribute of the <stanza-id/> element wasn't being set correctly (it
  was just set to the localpart instead of the full JID, which is out of spec).
- This meant that Conversations (at least) wouldn't trust it, and would try to
  use the origin-id instead, which couldn't be looked up when a chat marker was
  sent to the bridge.
- This fixes that.
- (and adds another space to the put-url debug info)

stuff.lisp

@@ -437,7 +437,10 @@ WhatsXMPP represents users as u440123456789 and groups as g1234-5678."
                                (cxml:with-element "stanza-id"
                                  (cxml:attribute "xmlns" +unique-stanzas-ns+)
                                  (cxml:attribute "id" xmpp-id)
-                                 (cxml:attribute "by" group-localpart))
+                                 (cxml:attribute "by" (concatenate 'string
+                                                                   group-localpart
+                                                                   "@"
+                                                                   (component-name comp))))
                                (cxml:with-element "origin-id"
                                  (cxml:attribute "xmlns" +unique-stanzas-ns+)
                                  (cxml:attribute "id" wa-id)))