{AH} add test for count_coverage() with missing contig, see #610
{AH} add test for count_coverage() with missing contig, see #610

pysam-developers/pysam,pysam-developers/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,pysam-developers/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,kyleabeauchamp/pysam,pysam-developers/pysam