Update checklist-model.js
Update checklist-model.js

Fix for the issue (Type property can't be changed).
https://github.com/vitalets/checklist-model/issues/19
vitalets/checklist-model,karthik-suresh/checklist-model,wesleyliu/checklist-model,marcopadillo/checklist-model,vitalets/checklist-model,wesleyliu/checklist-model,marcopadillo/checklist-model,karthik-suresh/checklist-model