NTLM auth: small fix
NTLM auth: small fix

Do not send leading '\' when no domain is given. (Should actually be no
difference, but is cleaner.)

otris/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,otris/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp