bufio: improve NewReaderSize, Peek, and UnreadByte test coverage
bufio: improve NewReaderSize, Peek, and UnreadByte test coverage

R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/42990045

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang