Return source pointer '/data/attributes' if attribute was not supplied in payload

[JoeWoodward]

This PR fixes the source pointer when the payload is missing a validated attribute.

e.g.

class User
  validates :name, presence: true
end

class UsersController
  deserializable_resource :user

  def create
    user = User.new(user_params)
    if user.save
      render jsonapi: user
    else
      render jsonapi_errors: user.errors
    end 
  end

  private
  
  def user_params
    params.require(:user).permit(:name)
  end
end

if we post

{
  data: {
    attributes: {}
  }
}

We get back an error with an empty pointer

{
  "errors": [
    {
      "title": "Invalid name",
      "detail": "Name can't be blank",
      "source": {}
    }
  ],
  "jsonapi": {
    "version": "1.0"
  }
}

With this change we return the node where the attribute is missing from

{
  "errors": [
    {
      "title": "Invalid name",
      "detail": "Name can't be blank",
      "source": {
        "pointer": "/data/attributes"
      }
    }
  ],
  "jsonapi": {
    "version": "1.0"
  }
}

[JoeWoodward]

Should we override the error description if the pointer is missing? http://jsonapi.org/examples/#error-objects-source-usage the example here returns a description

"detail": "Missing data Member at document's top level."
which maybe makes more sense. Then supply the original error in the meta

i.e.

{
  "errors": [
    {
      "detail": "Missing `name` Member at document's /data/attributes level",
      "source": {
        "pointer": "/data/attributes"
      },
      "meta": {
        "detailed_error_message": {
          "title": "Invalid name",
          "detail": "Name cannot be blank"
        }
      }
    }
  ],
  "jsonapi": {
    "version": "1.0"
  }
}

Maybe the meta is overkill. Probably enough to just override the detail and remote the title