Always new object for EOL

https://github.com/markov2/perl5-Mail-Message/issues/23

The end-of-line setting logic appeared to be wrong: it always created clone objects, which is a waste of time.

The EOL algorithm is used to assure that all lines transmitted via SMTP are terminated by a CR/LF, not only a CR (mac) or LF (Linux).

Where the old implementation mistakenly never copies the original lines, and the improved implementation does copy, the impact on performance is negative. However: it takes only a millisecond per a few thousand lines, measured.