closes https://github.com/TryGhost/Zapier/issues/56
- fixes tag creation when creating posts with `tags: [{slug: 'new'}]` which should be supported
- assigning tags with only `{slug: 'new'}` was triggering our validation for the required `name` property then bubbling up to the `bookshelf-relations` library resulting in a 500 error
- the fix applied here is to set the `name` field to the same as the `slug` field if a name is not provided
