Allow policies to be injected into controllers.

In controller specs instead of relying on Pundit to instantiate the correct policy object allow it to be injected into the controller. More often than not a double is used in controller specs which means the policy cannot be inferred. This also allows us to double the policy to ensure that on a unit level the right methods are being called.

``` ruby
class PostsController < ApplicationController
  attr_writer :post
  helper_method :post

  def create
    authorize post

    post.save
    respond_with post
  end

  private

  def post
    @post ||= Post.new post_attributes
  end
end

describe PagesController do
  let(:policy) { double 'SomePolicy', create?: true }

  before do
    controller.policy = policy
  end

  it 'delegates authorization to policy' do
    expect(policy).to have_received(:create?)
  end
end
```