Update DocDown and Underscore.

Former-commit-id: 7744ef274a9d8b47975dc9f3e3283bd778bc5403

vendor/underscore/underscore.js

@@ -369,6 +369,7 @@
 
   // Return the number of elements in an object.
   _.size = function(obj) {
+    if (obj == null) return 0;
     return (obj.length === +obj.length) ? obj.length : _.keys(obj).length;
   };
 
@@ -379,6 +380,7 @@
   // values in the array. Aliased as `head` and `take`. The **guard** check
   // allows it to work with `_.map`.
   _.first = _.head = _.take = function(array, n, guard) {
+    if (array == null) return void 0;
     return (n != null) && !guard ? slice.call(array, 0, n) : array[0];
   };
 
@@ -393,6 +395,7 @@
   // Get the last element of an array. Passing **n** will return the last N
   // values in the array. The **guard** check allows it to work with `_.map`.
   _.last = function(array, n, guard) {
+    if (array == null) return void 0;
     if ((n != null) && !guard) {
       return slice.call(array, Math.max(array.length - n, 0));
     } else {
@@ -491,6 +494,7 @@
   // pairs, or two parallel arrays of the same length -- one of keys, and one of
   // the corresponding values.
   _.object = function(list, values) {
+    if (list == null) return {};
     var result = {};
     for (var i = 0, l = list.length; i < l; i++) {
       if (values) {