span: minor fix, additional asserts
span: minor fix, additional asserts

malcolmgreaves/factorie,factorie/factorie,patverga/factorie,asubbaswamy/factorie,strubell/factorie,digitalreasoning/factorie-scala-210,hlin117/factorie,joprice/factorie,Craigacp/factorie,pogren-oracle/factorie,zxsted/factorie,melisabok/factorie