Fix modified annotations

```
<field name="message-annotations" type="fields"/>
```

Prior to this commit integration tests succeeded because both Erlang
client and RabbitMQ server contained a bug.

This bug was noticed by a Java client test suite.

deps/amqp10_client/src/amqp10_client_session.erl

@@ -1171,16 +1171,13 @@ make_link_ref(Role, Session, Handle) ->
     #link_ref{role = Role, session = Session, link_handle = Handle}.
 
 translate_message_annotations(MA)
-  when is_map(MA) andalso
-       map_size(MA) > 0 ->
-    Content = maps:fold(fun (K, V, Acc) ->
+  when map_size(MA) > 0 ->
+    {map, maps:fold(fun(K, V, Acc) ->
                             [{sym(K), wrap_map_value(V)} | Acc]
-                        end, [], MA),
-    #'v1_0.message_annotations'{content = Content};
+                    end, [], MA)};
 translate_message_annotations(_MA) ->
     undefined.
 
-
 wrap_map_value(true) ->
     {boolean, true};
 wrap_map_value(false) ->

deps/rabbit/src/rabbit_amqp_session.erl

@@ -1912,13 +1912,15 @@ settle_op_from_outcome(#'v1_0.modified'{delivery_failed = DelFailed,
                                         undeliverable_here = UndelHere,
                                         message_annotations = Anns0}) ->
     Anns = case Anns0 of
-               #'v1_0.message_annotations'{content = C} ->
-                   Anns1 = lists:map(fun({{symbol, K}, V}) ->
+               undefined ->
+                   #{};
+               {map, KVList} ->
+                   Anns1 = lists:map(
+                             %% "all symbolic keys except those beginning with "x-" are reserved." [3.2.10]
+                             fun({{symbol, <<"x-", _/binary>> = K}, V}) ->
                                      {K, unwrap(V)}
-                                     end, C),
-                   maps:from_list(Anns1);
-               _ ->
-                   #{}
+                             end, KVList),
+                   maps:from_list(Anns1)
            end,
     {modify,
      default(DelFailed, false),