BUG: #7757 Fix CSV parsing of singleton list header
 - [ ] closes #7757
 - [ ] added ```test_singleton_header``` in ```pandas/tests/io/parser/header.py``` / passed
 - [ ] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [ ] what's new entry

I fixed an issue with CSV parsing where passing a single-row header as a list to ```pd.read_csv```
would result in a ```pd.DataFrame``` of all ```NaN``` values. I added a simple parser test of whether there are any ```NaN``` values in the ```pd.DataFrame``` returned by ```pd.read_csv``` given a header list of length one.