archive/tar: add Copyright notice to example_test.go
archive/tar: add Copyright notice to example_test.go

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://codereview.appspot.com/7278050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go