AuthGssApiWithMic: Use default client creds instead of remote username (

#743)

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.

Co-authored-by: Jeroen van Erp <jeroen@hierynomus.com>

src/main/java/net/schmizz/sshj/userauth/method/AuthGssApiWithMic.java

@@ -84,8 +84,7 @@ public InitializeContextAction(Oid selectedOid) {
 
         @Override
         public GSSContext run() throws GSSException {
-            GSSName clientName = manager.createName(params.getUsername(), GSSName.NT_USER_NAME);
-            GSSCredential clientCreds = manager.createCredential(clientName, GSSContext.DEFAULT_LIFETIME, selectedOid, GSSCredential.INITIATE_ONLY);
+            GSSCredential clientCreds = manager.createCredential(GSSCredential.INITIATE_ONLY);
             GSSName peerName = manager.createName("host@" + params.getTransport().getRemoteHost(), GSSName.NT_HOSTBASED_SERVICE);
 
             GSSContext context = manager.createContext(peerName, selectedOid, clientCreds, GSSContext.DEFAULT_LIFETIME);