Issue #150: _.bind should allow binding functions to falsy values.

test/functions.js

@@ -20,10 +20,15 @@ $(document).ready(function() {
 
     var func = _.bind(func, this, 'curly');
     equals(func(), 'hello: curly', 'the function was completely applied in advance');
-    
+
     var func = function(salutation, firstname, lastname) { return salutation + ': ' + firstname + ' ' + lastname };
     func = _.bind(func, this, 'hello', 'moe', 'curly');
     equals(func(), 'hello: moe curly', 'the function was partially applied in advance and can accept multiple arguments');
+
+    func = function(context, message) { equals(this, context, message); };
+    _.bind(func, 0, 0, 'can bind a function to `0`')();
+    _.bind(func, '', '', 'can bind a function to an empty string')();
+    _.bind(func, false, false, 'can bind a function to `false`')();
   });
 
   test("functions: bindAll", function() {

underscore.js

@@ -412,7 +412,7 @@
     if (nativeBind && func.bind === nativeBind) return func.bind.apply(func, slice.call(arguments, 1));
     var args = slice.call(arguments, 2);
     return function() {
-      return func.apply(obj || {}, args.concat(slice.call(arguments)));
+      return func.apply(obj, args.concat(slice.call(arguments)));
     };
   };