Don't send HTML on post save.

fixes #3956
- adjusts post serialiser to not send HTML content
my 2c: `delete`, because `html:''` implies an empty, but valid property that could be used to overwrite the stored value.

When it comes to validation I think that it would make sense to require markdown OR html. With that in mind we could also talk about the benefits of changing the object to use `data:'your content'` and `type:'markdown'` at some point in the future.