py3: fix headencode() with display=False
We previously called str() on a email.header.Header object. On Python 2,
this returns a bytestring and the __str__ method is actually an alias to
.encode() method. On Python 3, __str__ does not perform encoding (and
returns a unicode string). To keep a consistent behavior across Python
versions, we explicitly use .encode() and we wrap the result with
encoding.strtolocal() to get a bytestring in all cases. As a side effect
of forcing bytes conversion, we need to decode back in _addressencode().
This is to make test-notify.t pass on Python 3.
Also note that headers are now encoded in some patchbomb tests; this is
because the charset is not always "us-ascii" ("iso-8859-1" otherwise) on
Python 3.
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