BUG: sniffing a csv raises with only a header
Add test to verify that #7773 is no longer an issue.  Closes #7773.

don't show headers that don't give any info as that requires people to click on it. Its much better to have something that is readable.
 lgtm. ping when green.
 @jreback : Travis has finally given the green light!  Ready to merge if there is nothing else.
 ty
