Fix #64166: quoted-printable-encode stream filter incorrectly discarding whitespace

If trailing whitespace on a line is detected, mark the linebreak as a
soft linebreak.
Fix #64166: quoted-printable-encode stream filter incorrectly discarding whitespace

Second attempt: need to use lookaheadto determine whether to encode ws
trailing ws
Move unsigned char declaration to top of block
add more tests for bug #64166
news for #64166