fread should distinguish between fill set explicitly or not set

Currently the default value for `fill` is `False`. However when `sep = ' '` we change `fill` to `True`. This shouldn't be the case if the user asked `fill=False` explicitly.

This FR proposes that default value for `fill` was `None`, and then `fread` can auto-detect the most appropriate value for this parameter.

See also `test_fread_issues.py::test_issue_R2535x`.