Fixed wrong behavior while using jQuery.noConflict and added test

While working on my own project I noticed what iframeResizer fails to initialize if the host page uses renames `jQuery` object using `jQuery.noConflict(true)`. That was happening because `jQuery` object remains present in `window`, but gets undefined value. Fixed that by changing the condition which checks if `jQuery` present on the page. Also, added the test for this case.