Pass the correct data to gdk_content_register_serializer()

The argument is eventually passed to g_conv(), so it should
be the charset, not the mime/type. Without this change the
contentype converter will fail to convert UTF-8 strings to, say,
CP-1251 later on.

gdk/gdkcontentserializer.c

@@ -895,8 +895,8 @@ init (void)
       gdk_content_register_serializer (G_TYPE_STRING,
                                        mime,
                                        string_serializer,
-                                       mime,
-                                       g_free);
+                                       (gpointer) charset,
+                                       NULL);
     }
   gdk_content_register_serializer (G_TYPE_STRING,
                                    "text/plain",