avoid negative test
The code was passing in the wrong value for userPrincipalName

userPrincipalName is a email like string such as "kkawaguchi@cloudbees.com", which is different from sAMAccountName like "kkawaguchi".

b4bfb5723677f1283bd8bbf48576c0025dd09726 appears to be responsible for this change, which could have been just a mistake.

Reference: http://msdn.microsoft.com/en-us/library/ms680857(v=vs.85).aspx