Update writer unit tests to test call data equality

Samples written by the writer should have the exact same data before and after
they are parsed. Previously this was not tested, since call data equality
testing only checks for the sample name, genotype, and record (and not other
data fields).
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.
Merge pull request #88 from bow/patch_writer-no-gt-field

Patch for Writer so it keeps non-GT data when writing records without GT data.