OAuht2 mock unit test when requesting authz

Add unit test to AGRestAuthzModuleSpec.m
Difficulties are:
1. to find a way to mock
requestAccess:(NSDictionary*) extraParameters
success:(void (^)(id object))success
failure:(void (^)(NSError *error))failure
which called:
[[UIApplication sharedApplication] openURL:[request URL]];

2. find a way to test private method like

exchangeAuthorizationCodeForAccessToken:(NSString*)code
success:(void (^)(id object))success
failure:(void (^)(NSError *error))failure