Stackoverflow in IE7 when _toString is called without any parameters

```
What steps will reproduce the problem?
1. In IE7 call date.toString() without any format parameter
2. IE7 will report a stackoverflow
3.

What is the expected output? What do you see instead?
- A formatted date

What version of the product are you using? On what operating system?
- On a trunk version of datejs from 3/5/2009 and IE7 on windows vista

Please provide any additional information below.
- The YUI calendar control is storing a date inside of an object property.
When that is done on IE7, it seems to be calling an implicit .toString() on
the date object.

When this is done the _toString function defined in core.js gets into a
recurive loop and keeps going until a stackoverflow happens.

There should be some sort of default format or a way to not get into a
recursive loop.

```

Original issue reported on code.google.com by `jeff.pen...@gmail.com` on 5 Mar 2009 at 7:24