Added that rendering will automatically insert the etag header on 200 OK responses. The etag is calculated using MD5 of the response body. If a request comes in that has a matching etag, the response will be changed to a 304 Not Modified and the response body will be set to an empty string. [DHH]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@6158 5ecf4fe2-1ee6-0310-87b1-e25e094e27de