mod_caps: Don't forget caps on XEP-0198 resumption

Many thanks to Thilo Molitor for spotting the issue and testing the fix.

src/mod_caps.erl

@@ -49,7 +49,8 @@
 	 handle_cast/2, terminate/2, code_change/3]).
 
 -export([user_send_packet/1, user_receive_packet/1,
-	 c2s_presence_in/2, mod_opt_type/1, mod_options/1, mod_doc/0]).
+	 c2s_presence_in/2, c2s_copy_session/2,
+	 mod_opt_type/1, mod_options/1, mod_doc/0]).
 
 -include("logger.hrl").
 
@@ -274,6 +275,13 @@ c2s_presence_in(C2SState,
 	    C2SState#{caps_resources => NewRs}
     end.
 
+-spec c2s_copy_session(ejabberd_c2s:state(), ejabberd_c2s:state())
+      -> ejabberd_c2s:state().
+c2s_copy_session(C2SState, #{caps_resources := Rs}) ->
+    C2SState#{caps_resources => Rs};
+c2s_copy_session(C2SState, _) ->
+    C2SState.
+
 -spec depends(binary(), gen_mod:opts()) -> [{module(), hard | soft}].
 depends(_Host, _Opts) ->
     [].
@@ -304,6 +312,8 @@ init([Host|_]) ->
 		       caps_stream_features, 75),
     ejabberd_hooks:add(s2s_in_post_auth_features, Host, ?MODULE,
 		       caps_stream_features, 75),
+    ejabberd_hooks:add(c2s_copy_session, Host, ?MODULE,
+		       c2s_copy_session, 75),
     ejabberd_hooks:add(disco_local_features, Host, ?MODULE,
 		       disco_features, 75),
     ejabberd_hooks:add(disco_local_identity, Host, ?MODULE,
@@ -341,6 +351,8 @@ terminate(_Reason, State) ->
 			  ?MODULE, caps_stream_features, 75),
     ejabberd_hooks:delete(s2s_in_post_auth_features, Host,
 			  ?MODULE, caps_stream_features, 75),
+    ejabberd_hooks:delete(c2s_copy_session, Host, ?MODULE,
+			  c2s_copy_session, 75),
     ejabberd_hooks:delete(disco_local_features, Host,
 			  ?MODULE, disco_features, 75),
     ejabberd_hooks:delete(disco_local_identity, Host,