GDBus: Use call() instead of invoke_method()

Lots of people been suggesting this. We still use MethodInvocation /
method_invocation for handling incoming method calls so use call()
instead of invoke_method() helps to separate the client and server
facilities. Which is a good thing(tm).

gio/tests/gdbus-introspection.c

@@ -53,13 +53,13 @@ introspection_on_proxy_appeared (GDBusConnection *connection,
   /*
    * Invoke Introspect(), then parse the output.
    */
-  result = g_dbus_proxy_invoke_method_sync (proxy,
-                                            "org.freedesktop.DBus.Introspectable.Introspect",
-                                            NULL,
-                                            G_DBUS_INVOKE_METHOD_FLAGS_NONE,
-                                            -1,
-                                            NULL,
-                                            &error);
+  result = g_dbus_proxy_call_sync (proxy,
+                                   "org.freedesktop.DBus.Introspectable.Introspect",
+                                   NULL,
+                                   G_DBUS_CALL_FLAGS_NONE,
+                                   -1,
+                                   NULL,
+                                   &error);
   g_assert_no_error (error);
   g_assert (result != NULL);
   g_variant_get (result, "(s)", &xml_data);