archive/tar: add test case for passing nil to FileInfoHeader
archive/tar: add test case for passing nil to FileInfoHeader

R=golang-codereviews, dave
CC=golang-codereviews
https://codereview.appspot.com/44710044

Committer: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

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