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

Lib/email/_header_value_parser.py

@@ -2789,6 +2789,7 @@ def _refold_parse_tree(parse_tree, *, policy):
             newline = _steal_trailing_WSP_if_exists(lines)
             if newline or part.startswith_fws():
                 lines.append(newline + tstr)
+                last_ew = None
                 continue
         if not hasattr(part, 'encode'):
             # It's not a terminal, try folding the subparts.