Previously the parts of the message retained whatever linesep they had on
read, which means if the messages weren't read in univeral newline mode, the
line endings could well be inconsistent. In general sending it via smtplib
would result in them getting fixed, but it is better to generate them
correctly to begin with. Also, the new send_message method of smtplib does
not do the fixup, so that method is producing rfc-invalid output without this
fix.
In Python2, if a unicode string was assigned as the value of a header,
email would automatically CTE encode it using the UTF8 charset.
This capability was lost in the Python3 translation, and this patch
restores it.
Patch by Ali Ikinci, assisted by R. David Murray.
I also added a fix for the mailbox test that was depending (with a comment
that it was a bad idea to so depend) on non-ASCII causing message_from_string
to raise an error. It now uses support.patch to induce an error during
message serialization.
Analogous to the decode_header fix, this fix makes Header.append and
make_header correctly handle the unknown-8bit charset introduced by email5.1,
when the input to them is binary strings. Previous to this fix the
make_header(decode_header(x)) == x invariant was broken in the face of the
unknown-8bit charset.
