Ignore PDU message lines in response when sending SMS

Tested with TP-LINK MA180.
It returns the PDU message when sending a message with a length greater
than 56 (i don't really know why for now). These lines are ignored and
only the last one with the status is parsed.

lib/biju/hayes.rb

@@ -113,7 +113,12 @@ module Biju
 
       if result[:prompt]
         modem.write("#{sms.to_pdu}#{26.chr}")
-        hayes_to_obj(modem.wait(length: 8).lstrip)
+        res = ''
+        loop do
+          res = modem.wait(length: 8)
+          break unless res.match(/\A[0-9A-Fa-f]+\r\n\z/)
+        end
+        hayes_to_obj(res.lstrip)
       end
     end