_.defaults only overrides undefined values

index.html

@@ -1215,7 +1215,7 @@ <h2 id="objects">Object Functions</h2>
       <p id="defaults">
         <b class="header">defaults</b><code>_.defaults(object, *defaults)</code>
         <br />
-        Fill in null and undefined properties in <b>object</b> with values from the
+        Fill in undefined properties in <b>object</b> with values from the
         <b>defaults</b> objects, and return the <b>object</b>. As soon as the
         property is filled, further defaults will have no effect.
       </p>

test/objects.js

@@ -92,12 +92,13 @@ $(document).ready(function() {
 
   test("defaults", function() {
     var result;
-    var options = {zero: 0, one: 1, empty: "", nan: NaN, string: "string"};
+    var options = {zero: 0, one: 1, empty: "", nan: NaN, nothing: null};
 
-    _.defaults(options, {zero: 1, one: 10, twenty: 20});
+    _.defaults(options, {zero: 1, one: 10, twenty: 20, nothing: 'str'});
     equal(options.zero, 0, 'value exists');
     equal(options.one, 1, 'value exists');
     equal(options.twenty, 20, 'default applied');
+    equal(options.nothing, null, "null isn't overriden");
 
     _.defaults(options, {empty: "full"}, {nan: "nan"}, {word: "word"}, {word: "dog"});
     equal(options.empty, "", 'value exists');

underscore.js

@@ -800,7 +800,7 @@
     each(slice.call(arguments, 1), function(source) {
       if (source) {
         for (var prop in source) {
-          if (obj[prop] == null) obj[prop] = source[prop];
+          if (obj[prop] === void 0) obj[prop] = source[prop];
         }
       }
     });