Inserting images with captions doesn't populate the image block caption and produces invalid block content

[afercia]

See (maybe) related #10594

On current master 4.0.0-rc.1:

• create a new post
• add an image block
• insert from the media library an image that already has a caption
• I'd expect the Image block caption to be populated with the caption text from the inserted image
• instead, the caption is empty:

• save the post as draft
• refresh the page
• the block now warns about invalid content:

I've tested also in Gutenberg 3.5 and the caption was populated correctly, so it appears to be a regression.

Changing the source from rich-text to html in processMediaCaption() as mentioned in #10594 makes the caption work again:

but I guess it's not a proper solution because:

• the caption placeholder doesn't disappear and it overlaps with the caption text
• after saving and refreshing the post, the "invalid content" warning still appears

[ellatrix]

Fixed by #10532.