[Content-blocks] Richtext content blocks content disappears on re-order

When you reorder new HTML content blocks, their content is wiped until you save.

To reproduce:

1. Check out the content blocks demo trunk (7.9)
2. Add

<properties>
<forge.content-blocks.version>1.11.00</forge.content-blocks.version>
</properties>

to the main pom
3. Create or open a document
4. Add an HTML block, add some text in it
5. Add another HTML block and some more text there
6. Move the first block down

Expected: the content blocks switch places

Actual: all content in both blocks is reverted to the prototype default ("body")