…rk if _SYSTEMCONFIGURATION_H is already defined
…works are missing, causing functionality to be missing or altered
…as the string value
…-data requests Parts for parameters with a value of NSNull now have an empty body rather than "<null>" (which was produced by -[NSNull description]).
…r. This enforces what the code comments say. Without this, Reachability might cause the application to crash on iOS 6 if the baseURL is nil.
…Agent" default header. closes AFNetworking/AFNetworking#542