Allow unlabelled columns

I'm importing a few files with badly formed headers, but which are always laid out the same way (they're automatically generated). Most columns are named, but a few aren't.

This PR both allows empty cells in the header row, and allows these unnamed columns to be imported by integer. Test suite passes on rspec <= 2.99, but no new tests added yet. If this functionality is desirable to anyone else, I'll add them.