tables.py: make more robust for  LongTable & spans
tables.py: make more robust for  LongTable & spans

kanarelo/reportlab,Distrotech/reportlab,kanarelo/reportlab,Distrotech/reportlab,kanarelo/reportlab,Distrotech/reportlab,Distrotech/reportlab,kanarelo/reportlab,kanarelo/reportlab,Distrotech/reportlab