Adapt regression test

Fix infinite loop in mb_encode_mimeheader

Implement mb_encode_mimeheader using fast text conversion filters

The behavior of the new mb_encode_mimeheader implementation closely
follows the old implementation, except for three poi

Implement mb_encode_mimeheader using fast text conversion filters

The behavior of the new mb_encode_mimeheader implementation closely
follows the old implementation, except for three points:

• The old implementation was missing a call to the mbfl_convert_filter
flush function. So it would sometimes truncate the input string just
before its end.

• The old implementation would drop zero bytes when QPrint-encoding.
So for example, if you tried to QPrint-encode the UTF-32BE string
"\x00\x00\x12\x34", its QPrint-encoding would be "=12=34", which
does not decode to a valid UTF-32BE string. This is now fixed.

• In some rare corner cases, the new implementation will choose to
Base64-encode or QPrint-encode the input string, where the old
implementation would have just added newlines to it. Specifically,
this can happen when there is a non-space ASCII character, followed
by a large number of ASCII spaces, followed by a non-ASCII character.

The new implementation is around 2.5-8x faster than the old one,
depending on the text encoding and transfer encoding used. Performance
gains are greater with Base64 transfer encoding than with QPrint
transfer encoding; this is not because QPrint-encoding bytes is slow,
but because QPrint-encoded output is much bigger than Base64-encoded
output and takes more lines, so we have to go through the process of
finding the right place to break a line many more times.

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