Custom Template fail reading header fields from email (inserts `??`)

[RealRaven2000]

When we try to create a new filter using a custom filter template the headers are not extracted as expected, instead they are replaced with the string ??.
This is probably a regression from recent changes to the classGetHeaders object.

[RealRaven2000]

This was auto-closed by a github commit. I will close it once I release to ATN.

[RealRaven2000]

Fixed in 5.9.2 - published 25/06/2023