Account for already inserted FWS

Application might have already inserted folding white space (FWS) into
header. Don't remove those already inserted FWSs, but account for them
accordingly by resetting line length count at those occurrences to fix
subsequent folding to happen if required and at expected position.

Note: we are assuming the already inserted LFs to be correct FWS
(i.e. LF SP), we don't care about broken ones.