image/png: use Scanner in reader_test.
image/png: use Scanner in reader_test.

R=nigeltao
CC=golang-dev
https://codereview.appspot.com/7399044

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