Fix DIGEST-MD5 support for picky servers

sleekxmpp/features/feature_mechanisms/stanza/auth.py

@@ -40,7 +40,7 @@ class Auth(StanzaBase):
         if not self['mechanism'] in self.plain_mechs:
             if values:
                 self.xml.text = bytes(base64.b64encode(values)).decode('utf-8')
-            else:
+            elif values == b'':
                 self.xml.text = '='
         else:
             self.xml.text = bytes(values).decode('utf-8')

sleekxmpp/util/sasl/mechanisms.py

@@ -467,7 +467,8 @@ class DIGEST(Mech):
             'qop': self.qop,
             'digest-uri': quote(self.digest_uri()),
             'response': self.response(b'AUTHENTICATE'),
-            'maxbuf': self.maxbuf
+            'maxbuf': self.maxbuf,
+            'charset': 'utf-8'
         }
         resp = b''
         for key, value in data.items():
@@ -480,7 +481,7 @@ class DIGEST(Mech):
             if self.cnonce and self.nonce and self.nonce_count and self.qop:
                 self.nonce_count += 1
                 return self.respond()
-            return b''
+            return None
 
         data = self.parse(challenge)
         if 'rspauth' in data: