Fallback to original implementation of plain auth_mechanism if socket is not provided

(cherry picked from commit 3bcdc0f359)

deps/rabbit/src/rabbit_auth_mechanism_plain.erl

@@ -40,8 +40,17 @@ handle_response(Response, #state{socket = Socket}) ->
             rabbit_access_control:check_user_login(User, AuthProps);
         error ->
             {protocol_error, "response ~tp invalid", [Response]}
+    end;
+
+handle_response(Response, _State) ->
+    case extract_user_pass(Response) of
+        {ok, User, Pass} ->
+            rabbit_access_control:check_user_pass_login(User, Pass);
+        error ->
+            {protocol_error, "response ~tp invalid", [Response]}
     end.
 
+
 build_auth_props(Pass, Socket) ->
     [{password, Pass}, {sockOrAddr, Socket}].