enhance #extend to allow taking multiple source objects.

This is done in jquery, extjs, and others as well.

added tests, no docs

test/objects.js

@@ -25,9 +25,14 @@ $(document).ready(function() {
   });
 
   test("objects: extend", function() {
-    var source = {name : 'moe'}, dest = {age : 50};
-    _.extend(dest, source);
-    equals(dest.name, 'moe', 'can extend an object with the attributes of another');
+    var result;
+    equals(_.extend({}, {a:'b'}).a, 'b', 'can extend an object with the attributes of another');
+    equals(_.extend({a:'x'}, {a:'b'}).a, 'b', 'properties in source override destination');
+    equals(_.extend({x:'x'}, {a:'b'}).x, 'x', 'properties not in source dont get overriden');
+    result = _.extend({x:'x'}, {a:'a'}, {b:'b'});
+    ok(_.isEqual(result, {x:'x', a:'a', b:'b'}), 'can extend from multiple source objects');
+    result = _.extend({x:'x'}, {a:'a', x:2}, {a:'b'});
+    ok(_.isEqual(result, {x:2, a:'b'}), 'extending from multiple source objects last property trumps');
   });
 
   test("objects: clone", function() {

underscore.js

@@ -432,10 +432,13 @@
     return _.filter(_.keys(obj), function(key){ return _.isFunction(obj[key]); }).sort();
   };
 
-  // Extend a given object with all of the properties in a source object.
-  _.extend = function(destination, source) {
-    for (var property in source) destination[property] = source[property];
-    return destination;
+  // Extend a given object with all the properties in given object(s)
+  _.extend = function(obj) {
+    var prop;
+    each(_.rest(arguments), function (source) {
+      for (prop in source) obj[prop] = source[prop];  
+    });
+    return obj;
   };
 
   // Create a (shallow-cloned) duplicate of an object.