vacation stumbles over invalid headers

[Militades Sunfire]

# 2.2.13: /etc/dovecot/dovecot.conf
# OS: Linux 3.16.0-4-amd64 x86_64 Debian 8.10

the vacation feature stumbles over invalid headers in several cases,
causing an even more invalid header

f.ex. if the original mail contains a line with only spaces in the
vacation reply the header breaks apart displying not only the inteded
vacation message but also parts of the message header. depends a bit on
the client what happends of course, thunderbird and the mac client are
two examples.

Greetings