BUG: Fix loadtxt with comments=None and a string None data.
BUG: Fix loadtxt with comments=None and a string None data.

Current loadtxt with `comments=None` considers the string `'None'` as a
comment symbol. Fixed by making split_line method check if comments is
None.

Closes #5155.

andsor/numpy,andsor/numpy,andsor/numpy,andsor/numpy