Memoize `parsed_body`.

It's common to use several assertions on the parsed response. The response
bodies aren't meant to be mutated. People should make new test requests
instead.

Thus, it should be safe to memoize the parsing.

actionpack/lib/action_dispatch/testing/test_response.rb

@@ -22,7 +22,7 @@ def self.from_response(response)
     attr_writer :response_parser # :nodoc:
 
     def parsed_body
-      @parsed_body ||= @response_parser.call(body)
+      @response_parser.call(body)
     end
   end
 end