Add fixes accidentally removed.

Yesterday, when improving how `parsed_body` extracted a parser I wrote
77bbf1e. Then I thought that was too many changes in one commit
and broke it up locally... or so I thought.

When pushed the extra commits removed the changes! Wups!

In shame, lob those changes together here:

  * 3b94c38 which meant to fix the CHANGELOG syntax error.
  * 5007df5 which meant to mention `parsed_body` in the docs.
  * 036a7a0 which meant to memoize the `parsed_body`.

2 files changed, 3 insertions, 2 deletions

diff --git a/actionpack/lib/action_dispatch/testing/integration.rb b/actionpack/lib/action_dispatch/testing/integration.rb
index 8a8e22053a..f4534b4173 100644
--- a/actionpack/lib/action_dispatch/testing/integration.rb
+++ b/actionpack/lib/action_dispatch/testing/integration.rb
@@ -729,7 +729,8 @@ module ActionDispatch
   #     response_parser: -> body { body }
   #
   # Where `param_encoder` defines how the params should be encoded and
-  # `response_parser` defines how the response body should be parsed.
+  # `response_parser` defines how the response body should be parsed through
+  # `parsed_body`.
   #
   # Consult the Rails Testing Guide for more.
 
diff --git a/actionpack/lib/action_dispatch/testing/test_response.rb b/actionpack/lib/action_dispatch/testing/test_response.rb
index 4f289ad4b5..9d4b73a43d 100644
--- a/actionpack/lib/action_dispatch/testing/test_response.rb
+++ b/actionpack/lib/action_dispatch/testing/test_response.rb
@@ -22,7 +22,7 @@ module ActionDispatch
     attr_writer :response_parser # :nodoc:
 
     def parsed_body
-      @response_parser.call(body)
+      @parsed_body ||= @response_parser.call(body)
     end
   end
 end