Make the tests pass again

Some tests are broken due to API and other implementation changes. This
patch makes the tests in test_header_parsing.rb and test_message.rb pass
again. I did have to change one assertion in test_blank_header_lines,
since it assumed that extra whitespace between List-Unsubscribe entries
is removed, which it currently isn't.

This does not fix test_server.rb, however, as the implementation also
has issues. I guess server.rb was the initial attempt on a sup server,
which is probably obsolete anyhow. Maybe it should be deleted?

Also, adds a rake testing task, so the tests will hopefully be run more
often :-)