Wiring up the custom http headers for all types of requests

Now all types of requests can benefit on the custom HTTP Headers support.

#### Example: 
    NSDictionary *serviceConfigDict = @{
        kNXOAuth2AccountStoreConfigurationClientID: @"XXXX",
        kNXOAuth2AccountStoreConfigurationSecret: @"YYYY",
        kNXOAuth2AccountStoreConfigurationAuthorizeURL: [NSURL URLWithString:@"https://secure.example.com/oauth2/authorize"],
        kNXOAuth2AccountStoreConfigurationTokenURL: [NSURL URLWithString:@"https://secure.example.com/oauth2/access"],
        kNXOAuth2AccountStoreConfigurationRedirectURL: [NSURL URLWithString:@"myApp://MyUrlSchema"],
        kNXOAuth2AccountStoreConfigurationTokenRequestHTTPMethod: @"POST",
       kNXOAuth2AccountStoreConfigurationCustomHeaderFields : @{@"Content-Type" : @"application/x-www-form-urlencoded"}
    };
    
    [[NXOAuth2AccountStore sharedStore] setConfiguration:serviceConfigDict
                                          forAccountType:@"MyService"];