Handle TypeError when msg object contains email.header.Header() instances

Ngalim Siregarrequested to merge
nsiregar/mailman:chores/typeerror-when-header-contains-header-instance into master

This MR to handle when Message object contains email.header.Header() instances by stringify them

Fix Issue #881 (closed) and #882 (closed)

Edited by Ngalim Siregar

Merge request reports