Return the correct server response for the 'message' response field from the server.
Previously this would use the generic message corresponding to the response code, which isn't always accurate. Since we're already parsing the headers, we can simply return that directly (This also has the benefit that it's the same code for both Streams and Curl header handling).
Fixes #28919


git-svn-id: https://develop.svn.wordpress.org/trunk@29848 602fd350-edb4-49c9-b593-d223f7449a82