<meta> tag getting added to the WYSIWYG output

This is a follow-up to #2287805: Caption filter does not work with Quarter Size image format - or with floated captions.

I didn't notice this in my original review, but the 'panopoly_images_fix_captions' filter is adding the following to the top of any field using WYSIWYG:

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

This is added to the document before passing to DOMDocument to ensure UTF-8, but we need to make sure it's not included when serializing the document to HTML.