reset-wp-caption-width.js

function adjust_caption() {
	var captions = document.getElementsByClassName('wp-caption');
	for (var i = 0; i < captions.length; i++) {
		// set wrap to auto
		captions[i].style.width = 'auto';
		// will set the width of the p to the same as the image. comment out if you don't want/need
		captions[i].getElementsByTagName('p')[0].style.width = captions[i].getElementsByTagName('img')[0].width + 'px';
	 }
}