bpo-36520: Email header folded incorrectly (#13608)

* bpo-36520: reset the encoded word offset when starting a new
line during an email header folding operation

* 📜🤖 Added by blurb_it.

* bpo-36520: add an additional test case, and provide descriptive
comments for the test_folding_with_utf8_encoding_* tests

* bpo-36520: fix whitespace issue

* bpo-36520: changes per reviewer request -- remove extraneous
backslashes; add whitespace between terminating quotes and
line-continuation backslashes; use "bpo-" instead of
"issue #" in comments