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

gnanderson/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang