go.net/spdy: make use of MaxDataLength
go.net/spdy: make use of MaxDataLength

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7355043

ssor/go.net,polydawn-ports/go.net,gigq/go.net,fernandez14/go.net,go-mirrors/go.net,andrewzeneski/go.net