extend mail header test to text mail (#13482)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11719 e93f8b46-1217-0410-a6f0-8f06a7374b81
add mail footer test (#13482)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11720 e93f8b46-1217-0410-a6f0-8f06a7374b81
not add empty header/footer to notification emails (#13482)

Contributed by Max Horn.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11721 e93f8b46-1217-0410-a6f0-8f06a7374b81