Fix bugs in overwriting the cell type
Fix bugs in overwriting the cell type

jupyter/nbgrader,jupyter/nbgrader,modulexcite/nbgrader,jhamrick/nbgrader,dementrock/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,alope107/nbgrader,alope107/nbgrader,jdfreder/nbgrader,MatKallada/nbgrader,modulexcite/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,jdfreder/nbgrader,jupyter/nbgrader,jhamrick/nbgrader,ellisonbg/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,jupyter/nbgrader,MatKallada/nbgrader,dementrock/nbgrader