Convert to_json to as_json to follow Ruby/Rails convention

[chadwilken]

Presently we are using to_json to render the JsonRenderable class to a Hash representation of the object (useful for storing in the DB for example). That doesn't follow the convention though where to_json should return the JSON string representation of the object. Instead we should be using as_json or to_h. I opted to implement as_json and alias to_h to that method.