docs, link to HTTP and CGI header definitions. [ci skip]

This was a reaction to: https://github.com/rails/rails/pull/9700#issuecomment-34550210

actionpack/test/dispatch/rack_test.rb

@@ -119,7 +119,7 @@ class RackRequestTest < BaseRackTest
     assert_equal "[2001:1234:5678:9abc:def0::dead:beef]", @request.host
   end
 
-  test "cgi environment variables" do
+  test "CGI environment variables" do
     assert_equal "Basic", @request.auth_type
     assert_equal 0, @request.content_length
     assert_equal nil, @request.content_mime_type

guides/source/testing.md

@@ -518,8 +518,10 @@ You also have access to three instance variables in your functional tests:
 
 ### Setting Headers and CGI variables
 
-Headers and cgi variables can be set directly on the `@request`
-instance variable:
+[HTTP headers](http://tools.ietf.org/search/rfc2616#section-5.3)
+and
+[CGI variables](http://tools.ietf.org/search/rfc3875#section-4.1)
+can be set directly on the `@request` instance variable:
 
 ```ruby
 # setting a HTTP Header