Media (WYSIWYG) / File entity css classes missing

Problem/Motivation

Currently it seems like some of the information provided by media_wysiwyg_token_to_markup(), which invokes theme_picture_formatter() isn't used to build the output.
This leads to missing attributes like css classes.

I'm not sure if this just applies to media_wysiwyg_token_to_markup() or for all the media / file entity output handling.

Proposed resolution

Check if $item has a class key and if so merge it into the attributes of $responsive_image.

Attention this likely changes the output on existing installations.

Remaining tasks

Reviews needed.

User interface changes

None.

API changes

None.

Data model changes

None.