Previously, AuthGssApiWithMic used params.getUsername() to create the
local client credential object. However, at least when using the native
GSS libraries (sun.security.jgss.native=true), the username would need 
to be something like "user@EXAMPLE.COM", not "user", or the library is 
unable to find credentials. Also, your remote username might not be your
local username.

Instead, and more simply, call the GSSManager#createCredential variant
that just uses default credentials, which should handle both of these 
cases.

Tested on Windows using SSPI. I haven't tested this patch on Linux but I
have confirmed that this form of call to createCredential works as I 
expect when using the native GSS/Kerberos library there too.