bytes.Buffer: return error in WriteTo if buffer is not drained
bytes.Buffer: return error in WriteTo if buffer is not drained

R=rsc
CC=golang-dev
http://codereview.appspot.com/5642065

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang