`ChangeHeader` index values are for differeniating between headers with
the same name. They are not the absolute index position of the header
within the full header list. They are also 1-based.

https://pythonhosted.org/pymilter/milter_api/smfi_chgheader.html

`kilter.protocol` 0.4.0 provides a new member for this.

The name of the produced package is correct and should be left alone.

This will be of use in an upcoming change…

This removes some of the boilerplate associated with MockMessageStream

This will stop filters without stopping the entire runner.

This should make no difference to current code, but may help defend
against future breakages.

Closes #10