Add `data` attribute comparison to _Call.__eq__
Add test case for VCF file without GT field values
Fix bug that removes sample data when GT field is not present

Some programs (e.g. bcftools) may output VCF files whose samples do not have its
GT field value. When dealing with files like these, PyVCF's writer will
(previously) remove all non-GT data and replace it with './.' since the
`_format_sample` function immediately returns upon failing to find GT data.

This fix addresses the issue, so that the Writer keeps any non-GT data intact.