Now set the prefix in the messageHeaders function

We add the prefix to the subject in the messageHeaders function. This
way we do not have to alter the value of the subject line in the cache.
There was a logic error in the last modification!

I have hopefully fixed the logic. If I understood the program flow in
PHPlist better, I could probably simplify even more.
Streamlined code with an important typo fixed

I now understand PHPlist a little better. So I have been able to remove
a lot of unnecessary code.
Added a couple of comments
Merge branch 'version-1.2a1'