Add tests to check parsed data validity

[k0d3r1s]

Hi. As I was trying to find an issue I fixed in #4, I found that even if I changed the code, all tests still were green even if data I got from my xlsx sheet changed.
This leaves a question - why?
I have a sheet with 105 columns and 41 rows. Nothing too extreme. But before #4 fix, I got back only 80 columns instead of 105.
I think you should add a test for something like this. I can see that current tests check for actual row or returned row count, while there are no checks about column count within rows

[simpletoimplement]

Will see what can I do about this. In meanwhile, PRs are welcome :)