`Session.headers.delete` not working

From Alistair MacDonald:

I'm wondering if there is an issue with the header removal logic. The docs talk about using session.headers.remove, and whilst this is currently session.headers.delete it doesn't seem to work. The pymilter smfi docs imply that a NULL value would be needed, but the code seems to be using an empty string. Of course, since I'm using Postfix rather than Sendmail this could also just be an issue with Postfix's milter implementation.

On investigation this is a problem with header index values.