Extract magic string "Policy" into a constant.

See #233

lib/pundit.rb

@@ -7,6 +7,8 @@ require "active_support/core_ext/module/introspection"
 require "active_support/dependencies/autoload"
 
 module Pundit
+  SUFFIX = "Policy"
+
   class NotAuthorizedError < StandardError
     attr_reader :query, :record, :policy
 

lib/pundit/policy_finder.rb

@@ -53,7 +53,7 @@ module Pundit
         else
           object.class
         end
-        "#{klass}Policy"
+        "#{klass}#{SUFFIX}"
       end
     end
   end