Apply the cache strategy for all policy lookups

This should be backwards-compatible because:

* `Pundit.policy` and `Pundit.policy!` are unaffected by this, since they use a non-cache cache.
* `Pundit::Context` is new, so nobody is relying on these methods yet.
* `Pundit::Authorization#policy` already cached the policy anyway.
* `policies` is explicitly discouraged, but even if it is relied upon it works as it always has, by default.

This is an improvement because we can change the cache strategy to introduce new features without breaking backwards-compatibility.

lib/pundit/authorization.rb

@@ -106,9 +106,9 @@ def policy_scope(scope, policy_scope_class: nil)
     #
     # @see https://github.com/varvet/pundit#policies
     # @param record [Object] the object we're retrieving the policy for
-    # @return [Object, nil] instance of policy class with query methods
+    # @return [Object] instance of policy class with query methods
     def policy(record)
-      policies[record] ||= pundit.policy!(record)
+      pundit.policy!(record)
     end
 
     # Retrieves a set of permitted attributes from the policy by instantiating

lib/pundit/context.rb

@@ -27,9 +27,7 @@ def authorize(possibly_namespaced_record, query:, policy_class:)
       policy = if policy_class
         policy_class.new(user, record)
       else
-        policy_cache.fetch(possibly_namespaced_record) do
-          policy!(possibly_namespaced_record)
-        end
+        policy!(possibly_namespaced_record)
       end
 
       raise NotAuthorizedError, query: query, record: record, policy: policy unless policy.public_send(query)
@@ -86,10 +84,7 @@ def policy_scope!(scope)
     # @raise [InvalidConstructorError] if the policy constructor called incorrectly
     # @return [Object, nil] instance of policy class with query methods
     def policy(record)
-      policy = policy_finder(record).policy
-      policy&.new(user, pundit_model(record))
-    rescue ArgumentError
-      raise InvalidConstructorError, "Invalid #<#{policy}> constructor is called"
+      cached_policy(record, &:policy)
     end
 
     # Retrieves the policy for the given record. Raises if not found.
@@ -101,14 +96,24 @@ def policy(record)
     # @raise [InvalidConstructorError] if the policy constructor called incorrectly
     # @return [Object] instance of policy class with query methods
     def policy!(record)
-      policy = policy_finder(record).policy!
-      policy.new(user, pundit_model(record))
-    rescue ArgumentError
-      raise InvalidConstructorError, "Invalid #<#{policy}> constructor is called"
+      cached_policy(record, &:policy!)
     end
 
     private
 
+    def cached_policy(record, &)
+      policy_cache.fetch(record) do
+        policy = yield policy_finder(record)
+        next unless policy
+
+        begin
+          policy.new(user, pundit_model(record))
+        rescue ArgumentError
+          raise InvalidConstructorError, "Invalid #<#{policy}> constructor is called"
+        end
+      end
+    end
+
     def policy_finder(record)
       PolicyFinder.new(record)
     end