FIX requestSerializer NSURLSessionConfiguration HTTPAdditionalHeaders

Default initialization of requestSerializer generates default headers "Accept-Language" and "User-Agent".
I'm using NSURLSessionConfiguration and this 2 two parameters are not overrided if they are already set in the request.

"If the same header appears in both this array and the request object (where applicable), the request object’s value takes precedence."

(according to the documentation : https://developer.apple.com/library/IOs/documentation/Foundation/Reference/NSURLSessionConfiguration_class/index.html#//apple_ref/occ/instp/NSURLSessionConfiguration/HTTPAdditionalHeaders)