Add ability to request the new object via API when calling build

This PR adds the ability to request the new object via the API when calling build. Just set the ``request_new_object_on_build`` flag and voilà, there you go!

#### Example:
```ruby
class Comment
  include Her::Model
  request_new_object_on_build true
end

@user = User.find(1)
@user.comments.build(body: "Just a draft")
# GET "/users/1/comments/new" with `body=Just+a+draft.`
# => [#<Comment id=nil body="Just a draft" archived=false user_id=1>]
```