decryptPostSave: true does not work if field is immutable

[jinusean]

A schema with field options immutable: true will return the field as undefined on save even if decryptPostSave: true.

The field is decrypted on find however.

[joegoldbeck]

Thanks for reporting this. Sounds like a bug!

Don't currently have any tests for the use of immutable - would be great to add them.

I don't have much time to work on this plugin these days, but am very happy to review any tested PRs :)