Fixes bug #129: Should not call __wakeup() on data which was created by Serializable::unserialize()
Fixes bug #126: Fatal error: "igbinary_serialize_zval: zval has unknown type 0" (IS_UNDEF)
Make this a warning instead of a fatal error (and serialize as null instead), since IS_UNDEF is a known type.
Later releases will fix the root cause of the warning, and consistently omit array/object/other entries for IS_UNDEF.
Compatible with PHP 5.2 - 7.1
Fixes crash in Memcached->setMulti (in php 7.0+) when the first level of array elements have references as values.
Other extensions using igbinary shouldn't be affected.