folded headers get corrupted

[ttoohey]

When the following message is sent to a mailhog service, the Subject line gets the Message-ID field appended to it (and mailhog generates a new Message-ID).

telnet localhost 1025 <<EOF
HELO test
MAIL FROM: <test@example.com>
RCPT TO: <test@example.com>
DATA
From: test@example.com
To: test@example.com
Subject: Testing
Message-ID:
 <message-id@localhost>

Body
.
QUIT
EOF

The subject appears as:

Subject: Testing <message-id@localhost>

I believe the following line is the source of the problem as it is expecting headers to always have a space after them:

data/message.go

Line 299 in 024d554

} else if strings.Contains(hdr, ": ") {

Is it incorrect behaviour to omit the space after the colon when folding? Is a space after the colon required even when no folding occurs?