Make SASLMechanism.getAuthenticationText() return byte[]

every SASL Mechanism is designed as a byte array based protocol. XMPP
adds the constraint that the challenges and responses are send base64
encoded. It's therefore better API design to let getAuthenticationText()
return byte[] instead of String.

smack-sasl-provided/src/main/java/org/jivesoftware/smack/sasl/provided/SASLDigestMD5Mechanism.java

@@ -62,7 +62,7 @@ public class SASLDigestMD5Mechanism extends SASLMechanism {
     }
 
     @Override
-    protected String getAuthenticationText() throws SmackException {
+    protected byte[] getAuthenticationText() throws SmackException {
         // DIGEST-MD5 has no initial response, return null
         return null;
     }