Restkit 0.20 - Change Authorization header with new token - ios

Question

I use Restkit in my iOS app to call an API.
When the app first launched, a client token is retrieved from the API in order to call the service to post a new user.
After this user has successfully being created, a new access token is sent back by the API. This time is it is a user token.
All the other requests to the API made by the app will now have to use this user token.
I am using a singleton class that inherits from RKObjectManager. I then built one class per ressource to access (example : Users, Images, ...) all inheriting from that main class called AKObjectManager.
In AKObjectManager I have the following method :
+ (instancetype)sharedManager
{
NSURL *url = [NSURL URLWithString:LLY_API_BASE_URL];
AKObjectManager *sharedManager = [self managerWithBaseURL:url];
sharedManager.requestSerializationMIMEType = RKMIMETypeJSON;
...
// Access Token
NSUserDefaults* userData = [NSUserDefaults standardUserDefaults];
if ([userData objectForKey:#"accessToken"]) {
// Not too sure if this is being taken into account for the other class that inherits
[ sharedManager.HTTPClient setDefaultHeader:#"Authorization" value:[userData objectForKey:#"accessToken"]];
}
return sharedManager;
}
I thought that by checking for every access the accessToken in NSUserDefaults and setting it in the Authorization field in the header would work but no. I can see through NSLog that the new access token is set when changing it for for some reasons using Charles the header of the request still points to the old one.
I then used
[[AKObjectManager sharedManager].HTTPClient setDefaultHeader:#"Authorization" value:accessToken.accessToken];
As soon as I got the new token but faced the same issue.
Finally I went for that road (UserManager inherits from AKObjectManager)
// Force the newly refresh token to be set in the Authorization header
[[UserManager sharedManager].HTTPClient setDefaultHeader:#"Authorization" value:[[NSUserDefaults standardUserDefaults] objectForKey:#"accessToken"]];
[[UserManager sharedManager] show:nil // userId equals nil meaning it will be replaced by 'self'
success:^(User* user){
self.user = user;
...
And it worked but I am not too happy about the implementation.
Could you point me to where I got it wrong and advise on how to do it ?