Fix #25; Skip empty continuation lines

[AdrienClairembault]

Q	A
Documentation	no
Bugfix	yes
BC Break	no
New Feature	no
RFC	no
QA	no

Description

The fix introduced in zendframework/zend-mail#196 allow a single empty continuation line in the headers.

The following example will be considered as valid:

From: user@example.com
To: userpal@example.net
Subject: This is your reminder
[SPACE]
  about the football game tonight 
Date: Wed, 20 Oct 2010 20:53:35 -0400 

But not these two:

From: user@example.com
To: userpal@example.net
Subject: This is your reminder
[SPACE]
  about the football 
[SPACE]
  game tonight 
Date: Wed, 20 Oct 2010 20:53:35 -0400 

From: user@example.com
To: userpal@example.net
Subject: This is your reminder
[SPACE]
[SPACE]
  about the football game tonight 
Date: Wed, 20 Oct 2010 20:53:35 -0400 

While the RFC indicate these empty continuation lines are no longer legal, it also recommend that they should just be ignored and not stop the message from being parsed:

The best handling of this example is for a message parsing engine to behave as if line {4}
were not present in the message

With these changes all empty continuation lines will be ignored so that the 3 example above will all be considered as valid and parsed accordingly.

[glensc]

@AdrienClairembault (for the future): avoid references in commit messages. Especially if you rebase and force push. These create reference noise for every push:

• Zend Mail does not conform with RFC2822 by not accepting content which contains additional extra lines. #25 (reference)

better keep ticket, commit, PR references in PR body itself.

[glensc]

@Slamdunk do you have a timeline or power to make this into releases? for this and #140?

[Slamdunk]

Yes I do, but I need to have a chat with @laminas TSC on how to maintain laminas repos, as I already saw myself having different opinion on how releases should be managed.
I have to say this will take time, sorry

[glensc]

Thanks, @Slamdunk! 🥳 🎉