Fix issue where supplying None for header_start causes read failure

#1844 introduced the possibility that explicitly providing `header_start=None` for a reader that already has that value for a default will cause the `read()` call to fail.  This was noticed in https://github.com/astropy/astroquery/issues/265.

The original motivation was that it should be an error to set `header_start` for readers like `FixedWidthNoHeader` because this is not meaningful.  However, we didn't anticipate setting it back to `None`, which is a do-nothing operation that should not cause failure.

cc @hamogu @keflavich