Use RSpec's described_class feature to specify the subject

Removes code duplication and makes it easier to rename policies.

README.md

@@ -501,7 +501,7 @@ Then put your policy specs in `spec/policies`, and make them look somewhat like
 
 ``` ruby
 describe PostPolicy do
-  subject { PostPolicy }
+  subject { described_class }
 
   permissions :update? do
     it "denies access if post is published" do

lib/generators/rspec/templates/policy_spec.rb

@@ -4,7 +4,7 @@ describe <%= class_name %>Policy do
 
   let(:user) { User.new }
 
-  subject { <%= class_name %>Policy }
+  subject { described_class }
 
   permissions ".scope" do
     pending "add some examples to (or delete) #{__FILE__}"

spec/policies/post_policy_spec.rb

@@ -4,7 +4,7 @@ describe PostPolicy do
   let(:user) { double }
   let(:own_post) { double(user: user) }
   let(:other_post) { double(user: double) }
-  subject { PostPolicy }
+  subject { described_class }
 
   permissions :update?, :show? do
     it "is successful when all permissions match" do