html/template: replace obsolete reference to template.Set Fixes issue 3053.
html/template: replace obsolete reference to template.Set
Fixes issue 3053.

R=golang-dev, dsymonds
CC=golang-dev
http://codereview.appspot.com/5656094

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go