Scopes on associations

Punch a hole in the association_proxy for scopes. Also inject the parents id if needed.

Addresses https://github.com/remiprev/her/issues/215
sorry I didn't make it clear. I'll switch the example from "comment" to "post", please bear with me for a moment.

```ruby
class User
  include Her::Model
  has_many :posts
end

class Post
  include Her::Model
  belongs_to :user
  collection_path '/users/:user_id/posts/' # specified collection_path

  scope :approved, -> { where(approved: true) }
end

user = User.find(1)
user.posts # => /users/1/posts

# works great
user.posts.approved # => /users/1/posts?approved=true

# but break this kind of usage
Post.all # => Her::Errors::PathError: Missing :_user_id parameter to build the request path. Path is `/users/:user_id/posts`. Parameters are `{}`
```

In web app we normally don't show a list of comment, they tend to attach to a specific user so specifying the `collection_path` totally makes sense. But things like Post we may also want to show a full list of them(`/posts`), and also posts that belong to a user(`/users/:user_id/posts`). I was seeking a way to make the "scopes on associations" thing work on this kind of usage.

---

If remove that `collection_path` from Post model

```ruby
user.posts # => /users/1/posts

# here user id got lost in the url path, but I really want it there
user.posts.approved # => /posts?approved=true
# but will allow this kind of usage
Post.all # => /posts
```