XEP-0480: Use base64 encoding for the salt

See: xsf/xeps#1403

specs/xmpp_codec.spec

@@ -5564,7 +5564,10 @@
                           enc = {enc_int, []},
                           dec = {dec_int, [1, infinity]},
                           required = true}],
-           result = {scram_upgrade_salt, '$iterations', '$cdata'}}).
+           cdata = #cdata{label = '$data',
+                          enc = {base64, encode, []},
+                          dec = {base64, decode, []}},
+           result = {scram_upgrade_salt, '$iterations', '$data'}}).
 
 -xml(scram_upgrade_hash,
      #elem{name = <<"hash">>,