HeaderCollection::Parse() trimmed the folding whitespace in front of every
continuation line starting with an encoded word. RFC 2047 6.2 ignores only
whitespace BETWEEN two adjacent encoded words; after a plain word, RFC 5322
unfolding keeps it. Symfony Mailer folds subjects exactly there, so
"de vos\r\n =?utf-8?Q?=C3=A9quipements?=" was displayed "voséquipements".
The message list (IMAP ENVELOPE) was fine; the opened message was not.
Test: php test/header-unfold.php (fails before the fix).
