Save embed_html field as unicode

The `embed_html` field is of type Text:

https://github.com/collective/sc.embedder/blob/73feddac53ccaa87108b061d42783584a9a98b2b/src/sc/embedder/content/embedder.py#L118

Text fields are saved as unicode by default. However, the `sc.embedder` form treats the data before saving and saves it as a string:

https://github.com/collective/sc.embedder/blob/73feddac53ccaa87108b061d42783584a9a98b2b/src/sc/embedder/content/embedder.py#L320

This is bad because it causes incompatibility with data saved programmatically, with the transmogrifier, which saves the data as unicode.