Add missing cast to SoftwareSerial::peek() when returning buffered value

Author: sandeepmistry

hardware/arduino/avr/libraries/SoftwareSerial/src/SoftwareSerial.cpp

@@ -482,5 +482,5 @@ int SoftwareSerial::peek()
     return -1;
 
   // Read from "head"
-  return _receive_buffer[_receive_buffer_head];
+  return (uint8_t)_receive_buffer[_receive_buffer_head];
 }