fixup! Improving parse function in modem class

Author: andreagilardoni

libraries/WiFiS3/src/Modem.cpp

@@ -203,7 +203,7 @@ ModemClass::ParseResult ModemClass::buf_read(const string &prompt, string &data_
             _serial_debug->print("<SP>");
          } else if(c < ' ') {
             _serial_debug->print("<");
-            _serial_debug->print((int)c);
+            _serial_debug->print((unsigned int)c);
             _serial_debug->print(">");
          } else {
             _serial_debug->print(c);