iproto: accept MP_STR *and* MP_BIN as type for authentication scramble

Accept MP_STR *and* MP_BIN as type for authentication scramble key
in IPROTO_AUTH packet.
Some codecs automatically pack non-utf-8 strings as MP_BIN. A scramble
is just an octect stream, so can (by chance) be both a utf-8 stream
and a binary stream, depending on the random seed.

src/box/authentication.cc

@@ -61,7 +61,18 @@ authenticate(const char *user_name, uint32_t len,
 			   "authentication request body");
 	}
 	mp_next(&tuple); /* Skip authentication mechanism. */
-	scramble = mp_decode_str(&tuple, &scramble_len);
+	if (mp_typeof(*tuple) == MP_STR) {
+		scramble = mp_decode_str(&tuple, &scramble_len);
+	} else if (mp_typeof(*tuple) == MP_BIN) {
+		/*
+		 * scramble is not a character stream, so some
+		 * codecs automatically pack it as MP_BIN
+		 */
+		scramble = mp_decode_bin(&tuple, &scramble_len);
+	} else {
+		tnt_raise(ClientError, ER_INVALID_MSGPACK,
+			   "authentication scramble");
+	}
 	if (scramble_len != SCRAMBLE_SIZE) {
 		/* Authentication mechanism, data. */
 		tnt_raise(ClientError, ER_INVALID_MSGPACK,