Fix respond_to with scopes

[jdelStrother]

I ran into problems trying to stub out sphinx scopes with rspec:

class Post
  sphinx_scope :public_posts, -> { { with: {public_read: true } }
end

allow(Post).to receive(:public_posts).and_return(double('sphinx-results'))

which raises "Post does not implement: public_posts". WDYT to this fix?

[pat]

Looks great, thank you :)

[pat]

Just published v4.2.0 which includes this PR :)