go spec: struct comparison only compares non-blank fields
go spec: struct comparison only compares non-blank fields

Fixes issue 3031.

R=golang-dev, rsc, r, iant
CC=golang-dev
http://codereview.appspot.com/5676054

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