is_serializable doesn't check for data created with a custom serialize

[jamesgol]

If someone uses a custom serialize as in
​http://php.net/manual/en/class.serializable.php

The serialized string will look like this

C:3:"obj":23:{s:15:"My private data";}

WordPress will happily serialize an object like this and store it in the options table, but on retrieve the raw string is returned. wp-includes/function.php:is_serialized() doesn't know about this type of string. Patch included to fix this

[sc0ttkclark]

Would love to see this fixed too.

[SergeyBiryukov]

Duplicate of #17375.