Fix a StackOverflowException when callback parameter is not present by reverting to default JSON Formatter

There are two other pull requests ([#8](https://github.com/WebApiContrib/WebApiContrib.Formatting.Jsonp/pull/8) and [#10](https://github.com/WebApiContrib/WebApiContrib.Formatting.Jsonp/pull/10)) that target the same StackOverFlowException fix. However, those requests don't honor the fact that it should be returning a request specific version of the formatter. This one does.

Also included is an example aspx page that shows the StackOverflowException in action. It probably doesn't belong here but it was an easy way for me to show the problem.

Whether or not this is the *right* way to handle the situation, I am not sure. It seems like, for the example request in the aspx file, the JSONP formatter should never have been called, but somehow it is. This fix at least lets you fail to JSON instead of taking down the entire server through a stack overflow.

Thanks for the great plug-in!