Ensure _.isEqual returns boolean values even if callback doesn't.

Former-commit-id: b2c31ee1711a436e5400c8e80c9f54a9680301b7

test/test.js

@@ -1262,6 +1262,16 @@
       var actual = _.isEqual('a', 'a', function() { });
       strictEqual(actual, true);
     });
+
+    test('should return a boolean value even if `callback` does not', function() {
+      var actual = _.isEqual('a', 'a', function() { return 'a'; });
+      strictEqual(actual, true);
+
+      _.each(falsey, function(value) {
+        var actual = _.isEqual('a', 'b', function() { return value; });
+        strictEqual(actual, false);
+      });
+    });
   }());
 
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