Update EmailMessage to leverage Django's mail header handling.
Review Request #11680 — Created June 25, 2021 and submitted
EmailMessageclass maintains its own list of headers to add during
message building. We were using
add_header()to do this, which
technically works, but bypasses Django's custom logic for handling
various encoding and address-related header situations.
This change switches to the preferred method,
msg[header] = ..., and
adds several new unit tests to check headers when Unicode characters are
Unit tests pass on Python 2 and 3.