Fixed null reference in WMSGetFeatureInfo on Internet Explorer 11

I ran into a small issue with the WMSGetFeatureInfo today. If I set the infoFormat option to 'application/json', then I end up with a null reference exception in format.read on Internet Explorer 11.  I thought that it might be better to have the handleResponse method check the content type of the response (and the type in infoFormat) to determine if the response is 'application/json'.  In the case that the response is 'application/json', parse the responseText using OpenLayers.Format.GeoJSON.

I have added this logic to the handleResponse method, and added an appropriate test case to the test harness.  Also, this is only my second contribution to an open source project, and my first pull request on GitHub, so any constructive criticism would be greatly appreciated.