Fixing cell_type in CodeCell constructor.

This was causing the cell_type to be null for CodeCells, which
broke the cell type dropdown in the toolbar.

IPython/html/static/notebook/js/codecell.js

@@ -63,7 +63,6 @@ var IPython = (function (IPython) {
     var CodeCell = function (kernel, options) {
         this.kernel = kernel || null;
         this.collapsed = false;
-        this.cell_type = "code";
 
         // create all attributed in constructor function
         // even if null for V8 VM optimisation
@@ -82,6 +81,9 @@ var IPython = (function (IPython) {
 
         IPython.Cell.apply(this,[options]);
 
+        // Attributes we want to override in this subclass.
+        this.cell_type = "code";
+
         var that = this;
         this.element.focusout(
             function() { that.auto_highlight(); }