Return JSON from contents API checkpoint methods

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

@@ -2341,7 +2341,6 @@ define([
      * @param {Object} data JSON representation of a checkpoint
      */
     Notebook.prototype.list_checkpoints_success = function (data) {
-        data = $.parseJSON(data);
         this.checkpoints = data;
         if (data.length) {
             this.last_checkpoint = data[data.length - 1];
@@ -2373,7 +2372,6 @@ define([
      * @param {Object} data JSON representation of a checkpoint
      */
     Notebook.prototype.create_checkpoint_success = function (data) {
-        data = $.parseJSON(data);
         this.add_checkpoint(data);
         this.events.trigger('checkpoint_created.Notebook', data);
     };

IPython/html/static/services/contents.js

@@ -183,6 +183,7 @@ define([
         var url = this.api_url(path, 'checkpoints');
         var settings = {
             type : "POST",
+            dataType : "json",
         };
         return utils.promising_ajax(url, settings);
     };
@@ -191,6 +192,8 @@ define([
         var url = this.api_url(path, 'checkpoints');
         var settings = {
             type : "GET",
+            cache: false,
+            dataType: "json",
         };
         return utils.promising_ajax(url, settings);
     };