Fix 'TODO: section 5.1: we should restrict the allowed headers to those headers that contain an "address-list"' issue in sifter\tests\address.py

[manfred-kaiser]

section 5.1: we should restrict the allowed headers to
those headers that contain an "address-list". this includes at
least: from, to, cc, bcc, sender, resent-from, resent-to.

CodeFactor found an issue: TODO: section 5.1: we should restrict the allowed headers to

It's currently on:
sifter\tests\address.py:66
Commit 6e6601e