Fixes Issue 54

[tychay]

Looks like WordPress core has a bug where get_image_tag() returns a different class than wp_get_attachment_image(). This shouldn't be a filter though, but patched in the plugin itself given the core spits it out.

Note to upstream: Without this patch, post thumbnails might not get properly sized (not sure they'll get properly sized post patch as I don't use this plugin yet). Even if they do get properly sized some parts of the code that are wasteful will get exercised to determine the srcset mappings.