Warning error occurs on the default parameter of "admin-head-callback" in add_theme_support.

[jim912]

When you make settings of custom header using add_theme_support on functions.php of a theme, if you use default value without defining admin-head-callback, warning error occurs as follows on admin screen of custom header.

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function '' not found or invalid function name in path-to-wordpress-root/wp-includes/plugin.php on line 403

To fix this, I think better to change default value of admin-head-callback into __return_false, on /wp-includes/theme.php. Here's patch.