made internalSubset default to null instead of '', added exception for importing a document type node.
made internalSubset default to null instead of '', added exception for importing a document type node.

jeffcarp/jsdom,medikoo/jsdom,darobin/jsdom,susaing/jsdom,AVGP/jsdom,AshCoolman/jsdom,fiftin/jsdom,singlebrook/linkck,oelmekki/jsdom,beni55/jsdom,kangax/jsdom,snuggs/jsdom,evdevgit/jsdom,kesla/jsdom,evanlucas/jsdom,Ye-Yong-Chi/jsdom,sirbrillig/jsdom,danieljoppi/jsdom,fiftin/jsdom,snuggs/jsdom,Zirro/jsdom,kesla/jsdom,cpojer/jsdom,zaucy/jsdom,danieljoppi/jsdom,Joris-van-der-Wel/jsdom,nicolashenry/jsdom,cpojer/jsdom,Joris-van-der-Wel/jsdom,darobin/jsdom,vestineo/jsdom,aduyng/jsdom,kittens/jsdom,mzgol/jsdom,iizukanao/jsdom,medikoo/jsdom,Sebmaster/jsdom,vestineo/jsdom,susaing/jsdom,isaacs/jsdom,zaucy/jsdom,sebmck/jsdom,sebmck/jsdom,kesla/jsdom,oelmekki/jsdom,jeffcarp/jsdom,janusnic/jsdom,papandreou/jsdom,lcstore/jsdom,selam/jsdom,matthewpflueger/jsdom,fiftin/jsdom,vestineo/jsdom,concord-consortium/jsdom,isaacs/jsdom,Sebmaster/jsdom,darobin/jsdom,AshCoolman/jsdom,mbostock/jsdom,selam/jsdom,crealogix/jsdom,kangax/jsdom,evanlucas/jsdom,Zirro/jsdom,concord-consortium/jsdom,sirbrillig/jsdom,robertknight/jsdom,selam/jsdom,dexteryy/jsdom-nogyp,evdevgit/jsdom,beni55/jsdom,zaucy/jsdom,substack/jsdom,iizukanao/jsdom,sebmck/jsdom,kevinold/jsdom,szarouski/jsdom,robertknight/jsdom,mbostock/jsdom,evanlucas/jsdom,AshCoolman/jsdom,evdevgit/jsdom,papandreou/jsdom,danieljoppi/jsdom,mzgol/jsdom,beni55/jsdom,tmpvar/jsdom,crealogix/jsdom,janusnic/jsdom,kittens/jsdom,janusnic/jsdom,nicolashenry/jsdom,AVGP/jsdom,szarouski/jsdom,kevinold/jsdom,Ye-Yong-Chi/jsdom,aduyng/jsdom,lcstore/jsdom,jeffcarp/jsdom,mbostock/jsdom,Sebmaster/jsdom,kittens/jsdom,godmar/jsdom,cpojer/jsdom,tmpvar/jsdom,substack/jsdom,godmar/jsdom,brianmcd/cloudbrowser-jsdom,sirbrillig/jsdom,susaing/jsdom,Ye-Yong-Chi/jsdom,medikoo/jsdom,mzgol/jsdom,szarouski/jsdom,dexteryy/jsdom-nogyp,robertknight/jsdom,brianmcd/cloudbrowser-jsdom,nicolashenry/jsdom,Joris-van-der-Wel/jsdom,aduyng/jsdom,kevinold/jsdom,matthewpflueger/jsdom,lcstore/jsdom