Fix attribute set undefined, transition super<->sub
Fix attribute set undefined, transition super<->sub

An "undefined" key was being adding to the attribute set because all
attribute sets do not define an "exclusiveWith" attribute.

Additionally, transitioning from superscript to subscript or vice-versa
was broken because the exclusive attribute was not being consistently
deleted.

ritzyed/ritzy,deepti2200/ritzy,deepti2200/ritzy,sangohan/ritzy,sangohan/ritzy,guiquanz/ritzy,yakovenkodenis/ritzy,yakovenkodenis/ritzy,deepti2200/ritzy,shaunstanislaus/ritzy,shaunstanislaus/ritzy,eiriklv/ritzy,lihan/ritzy,gaurav-rygbee/ritzy,eiriklv/ritzy,ritzyed/ritzy,guiquanz/ritzy,lihan/ritzy,guiquanz/ritzy,sangohan/ritzy,eiriklv/ritzy,gaurav-rygbee/ritzy,yakovenkodenis/ritzy,lihan/ritzy,gaurav-rygbee/ritzy,ritzyed/ritzy,shaunstanislaus/ritzy