Added proper checks for observing AFHTTPRequestSerializerObservedKeyPaths

This addresses issues #2284, #2288 and #2289
Merge pull request #2294 from jmkk/dynamic_keypaths_serialization

Added proper checks for observing AFHTTPRequestSerializerObservedKeyPaths
Squashed commit of the following:

commit 78b42ccb1798b9e4c67136f693002cdbb73b7417
Author: TimWatson <tewatson89@gmail.com>
Date:   Mon Sep 15 14:48:52 2014 -0700

    Removed extra parenthesis

commit c9195763295a6939cfc395d1a514500d5ff7d5be
Author: TimWatson <tewatson89@gmail.com>
Date:   Mon Sep 15 12:09:16 2014 -0700

    Switched User-Agent to use the version number instead of the build number

Signed-off-by: Mattt Thompson <m@mattt.me>
Merge branch 'master' of github.com:AFNetworking/AFNetworking
Adding test for HTTPS request
Merge branch 'master' of https://github.com/AFNetworking/AFNetworking