replace explicit xmlChar cast with BAD_CAST
replace explicit xmlChar cast with BAD_CAST

nickerso/libcellml,cellml/libcellml,hsorby/libcellml,MichaelClerx/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,nickerso/libcellml,MichaelClerx/libcellml,hsorby/libcellml,libcellml/buildbot-testing,libcellml/buildbot-testing,hsorby/libcellml,nickerso/libcellml