change boundary implementation to hex

[quinlanj]

The current implementation of the boundary doesn't make sense because we are generating a number between 0-9 and converting it to a string with a radix of 16.

If we want the boundary to be base 10, we should have:
boundary += Math.floor(Math.random() * 10).toString(10);

If we want the boundary to be base 16, we should have:
boundary += Math.floor(Math.random() * 16).toString(16);

I'd recommend base 16 because it has more entropy and still conforms with the rfc spec