io: add tests for SectionReader ReadAt and Size
io: add tests for SectionReader ReadAt and Size

R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/39200045

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang