Ensure proper cast of char to avoid hitting int overload

There's a bug in the sample code for this sample:
http://ms-iot.github.io/content/UART.htm

In the first line of the loop() method, this line would never return false, causing an error to be logged:
 if (Serial1.write(output) != 1)

The problem is that 'output' will be hitting the 'int' overload, and thus write 4 bytes rather than one. This means the return value is always 4.
Instead adding a cast makes the sample run correctly:
     if (s.write((uint8_t)output) != 1)