fixed setting of empty tags in AlignedRead
fixed setting of empty tags in AlignedRead

--HG--
branch : rel-0.3

pysam-developers/pysam,kyleabeauchamp/pysam,nlhepler/pysam,dannon/pysam,kyleabeauchamp/pysam,dannon/pysam,bioinformed/pysam,bioinformed/pysam,mattions/pysam,kyleabeauchamp/pysam,brendanofallon/pysam,brendanofallon/pysam,pysam-developers/pysam,nlhepler/pysam,bioinformed/pysam,carolinux/pysam,nlhepler/pysam,brendanofallon/pysam,brendanofallon/pysam,TyberiusPrime/pysam,TyberiusPrime/pysam,daler/pysam,carolinux/pysam,bioinformed/pysam,daler/pysam,TyberiusPrime/pysam,mattions/pysam,pysam-developers/pysam,bioinformed/pysam,nlhepler/pysam,dannon/pysam,daler/pysam,dannon/pysam,kyleabeauchamp/pysam,carolinux/pysam,nlhepler/pysam,TyberiusPrime/pysam,carolinux/pysam,carolinux/pysam,pysam-developers/pysam,TyberiusPrime/pysam,daler/pysam,mattions/pysam,brendanofallon/pysam,kyleabeauchamp/pysam,mattions/pysam,mattions/pysam,dannon/pysam,daler/pysam