kilter.protocol 0.6 makes a change that necessitates an assertion that
messages with bytes data exclusively use memoryviews, at least until
`freeze()` is called on them.

There should be a typing solution to this problem but it may have it's
own issues.

For real this time, hopefully. At least the tests pass now.

`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