Remove outdated comment in request.rb about instance-memoization


* Removed memoization in: 0bf8d8e840ae226d6efed8af345feb67a10f6aa6
* Added memoization + comment in: eefbed89c4ece749e889132012d0f67cd87926a8
Merge pull request #662 from johanlunds/patch-1

Cleanup: Remove outdated comment in request.rb about instance-memoization