direct_image throws TypeError

* The _raw method accepts only byte strings

escpos/escpos.py

@@ -251,7 +251,7 @@ class Escpos(object):
                     i = 0
                     temp = 0
         self._raw(binascii.unhexlify(bytes(buf, "ascii")))
-        self._raw('\n')
+        self._raw(b'\n')
 
     def qr(self, text):
         """ Print QR Code for the provided string