bufio: Write and WriteString cleanup
bufio: Write and WriteString cleanup

Write never writes less than the buffer size and WriteString takes advantage
of the copy built-in to improve write efficiency.

R=rsc, ality, rog
CC=golang-dev
http://codereview.appspot.com/4344060

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

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