HTML in image captions: improve converting the caption html elements to a...

HTML in image captions: improve converting the caption html elements to a shortcode, catch some rare cases where image with a caption is pasted in the visual editor, see #18311

git-svn-id: https://core.svn.wordpress.org/trunk@20013 1a063a9b-81f0-0310-95a4-ce76da25c4cd