Fix subclassing note comments in AFHTTPRequestOperationManager.h
[Issue #1335] Adding default authentication challenge handling in AFURLSessionManager

Signed-off-by: Mattt Thompson <m@mattt.me>
Merge branch '2.0' of github.com:AFNetworking/AFNetworking into 2.0
Merge pull request #1331 from raylillywhite/patch-2

Fix subclassing note comments in AFHTTPRequestOperationManager.h