gmt_offset is not set when timezone strings are used

[jessepearson]

Steps to reproduce

1. Create a clean install
2. Go to Settings > General
3. Change time zone to any string that is not UTC+-
4. View gmt_offset in wp_options, it will be empty

Notes

This appears to happen here:
https://core.trac.wordpress.org/browser/trunk/src/wp-admin/options.php#L177

If gmt_offset is empty, then current_time always returns UTC time.
https://core.trac.wordpress.org/browser/trunk/src/wp-includes/functions.php#L61

[jessepearson]

Came into this with fresh eyes this morning, it appears that this filter exists:

// Timezone
add_filter( 'pre_option_gmt_offset','wp_timezone_override_offset' );

This comes into play when get_option( 'gmt_offset' ) is used. It will convert the timezone string into a UTC offset and return it.