USB CDC available() method returns correct number of bytes in buffer.

See #1953

(Filtered from arduino/Arduino@6914af0)

Author: cmaglie

core/CDC.cpp

@@ -129,7 +129,7 @@ void Serial_::end(void)
 int Serial_::available(void)
 {
 	if (peek_buffer >= 0) {
-		return 1;
+		return 1 + USB_Available(CDC_RX);
 	}
 	return USB_Available(CDC_RX);
 }