Blog API does not pass along text formats correctly

This was discovered while working on #11218: Allow default text formats per role, and integrate text format permissions and at one point was necessary to make the patch there pass all tests, but now it isn't so I'm splitting it off to its own bug report :)

The problem is that the the validate and submit handlers in Drupal now expect node formats to be stored in the 'body_format' key, but the Blog API module is still passing them in the 'format' key.

An initial patch is attached, but as per a suggestion by @sun, we might want to consider storing things in the 'body_format' key only (and never using 'format'). However, there are some hooks that get fired before this, and it's not immediately clear to me which is the right one to send to them. The patch here therefore makes the minimal (but somewhat ugly) change.

We probably also should write a test for this.