Bug 1091295 - [Email] Failure to normalize newlines in composed messages. May cause some SMTP servers (ex: qmail) to refuse to send mail with a 451 error.

Note that r+-ing this is an implicit r+ of
https://github.com/mozilla-b2g/mail-fakeservers/pull/24
which I have speculatively already landed.  In that vein, note that I've also
updated our package.json to NOT use a tilde for mail-fakeservers since the
tilde regrettably defeats my whole concept of "we don't take the upgrade
without a gelam upgrade".
The [^\n] would actually match a character and eat it with the replace as currently happening. We do have lookahead stuff so we could do (?!\n) to assert \r not followed by \n.  But as you note, I think the greediness (and de facto standards stuff) has our back here.  I did run a test with various test variants and it turned out as desired.  Also, this is actually the same regexp being used by the mimelib quoted-printable encoding funcs, so it's what we've been using in v2.0 and earlier.  I perhaps should have called that out in the comments.  (I was trying to avoid using a cargo cult rationalizations in the code.)