* Added a unit test for this case.
* Added a TODO item about the protocol errors.
* Added SetUserFirst protocol error and new mapping to error
code 531 (protocol change!).
* Adapated Handler#pass_cmd to raise the SetUserFirst error
when a username has not been set.
git-svn-id: svn+ssh://svn.luon.net/svn/ildus/trunk@7 65a33f86-aa00-0410-91be-cd1bf5efb309