Make _.isEqual treat invalid dates as equiv.

lodash.js

@@ -5168,18 +5168,14 @@
 
         case boolTag:
         case dateTag:
-          // Coerce dates and booleans to numbers, dates to milliseconds and
+        case numberTag:
-          // booleans to `1` or `0` treating invalid dates coerced to `NaN` as
+          // Coerce booleans to `1` or `0` and dates to milliseconds.
-          // not equal.
+          // Invalid dates are coerced to `NaN`.
-          return +object == +other;
+          return eq(+object, +other);
 
         case errorTag:
           return object.name == other.name && object.message == other.message;
 
-        case numberTag:
-          // Treat `NaN` vs. `NaN` as equal.
-          return (object != +object) ? other != +other : object == +other;
-
         case regexpTag:
         case stringTag:
           // Coerce regexes to strings and treat strings, primitives and objects,

test/test.js

@@ -9777,9 +9777,10 @@
       var date = new Date(2012, 4, 23);
 
       assert.strictEqual(_.isEqual(date, new Date(2012, 4, 23)), true);
+      assert.strictEqual(_.isEqual(new Date('a'), new Date('b')), true);
+
       assert.strictEqual(_.isEqual(date, new Date(2013, 3, 25)), false);
       assert.strictEqual(_.isEqual(date, { 'getTime': lodashStable.constant(+date) }), false);
-      assert.strictEqual(_.isEqual(new Date('a'), new Date('a')), false);
     });
 
     QUnit.test('should compare error objects', function(assert) {

test/underscore.html

@@ -285,6 +285,7 @@
             'Commutative equality is implemented for `0` and `-0`',
             '`new Number(0)` and `-0` are not equal',
             'Commutative equality is implemented for `new Number(0)` and `-0`',
+            'Invalid dates are not equal',
             'false'
           ],
           'isFinite': [