root node in json should not be mandatory

[Rebolon]

For instance for a Book i must have a root book node like this:

{ 
  book: {
    ...
  }
}

This is useless and so it should be optional ! (ApiPlatform with Denormalizer doesn't need this root node)

[Rebolon]

Not only ApiPlatform doesn't need it, but Symfony Serializer too. So i have to remove it !